<?xml version="1.0" encoding="us-ascii"?>
<!-- DFIN ActiveDisclosure(SM) XBRL Linkbase Document - http://www.dfinsolutions.com/ -->
<!-- Creation Date      : 2022-05-03T23:44:48.3600791+00:00 -->
<!-- Version            : 5.0.1.747 -->
<!-- Package ID         : 6c07a7e750ca4f7ba2f7f048e70cc7e5 -->
<!-- Copyright (c) 2022 Donnelley Financial Solutions, Inc. All Rights Reserved. -->
<link:linkbase xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd">
  <link:arcroleRef xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all" arcroleURI="http://xbrl.org/int/dim/arcrole/all" />
  <link:arcroleRef xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default" arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" />
  <link:arcroleRef xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain" arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" />
  <link:arcroleRef xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member" arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" />
  <link:arcroleRef xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension" arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" />
  <link:roleRef xlink:type="simple" xlink:href="psn-20220331.xsd#TemplateLink" roleURI="http://www.parsons.com/20220331/role/TemplateLink" />
  <link:roleRef roleURI="http://www.parsons.com/20220331/taxonomy/role/StatementConsolidatedBalanceSheetsParentheticalUnaudited" xlink:href="psn-20220331.xsd#Role_StatementConsolidatedBalanceSheetsParentheticalUnaudited" xlink:type="simple" />
  <link:definitionLink xlink:type="extended" xlink:role="http://www.parsons.com/20220331/taxonomy/role/StatementConsolidatedBalanceSheetsParentheticalUnaudited">
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TreasuryStockShares" xlink:type="locator" xlink:label="us-gaap_TreasuryStockShares" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonStockSharesOutstanding" xlink:type="locator" xlink:label="us-gaap_CommonStockSharesOutstanding" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonStockSharesIssued" xlink:type="locator" xlink:label="us-gaap_CommonStockSharesIssued" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonStockSharesAuthorized" xlink:type="locator" xlink:label="us-gaap_CommonStockSharesAuthorized" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonStockParOrStatedValuePerShare" xlink:type="locator" xlink:label="us-gaap_CommonStockParOrStatedValuePerShare" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ContractWithCustomerLiabilityCurrent" xlink:type="locator" xlink:label="us-gaap_ContractWithCustomerLiabilityCurrent" />
    <link:loc xlink:href="psn-20220331.xsd#psn_AccruedExpensesAndOtherCurrentLiabilities" xlink:type="locator" xlink:label="psn_AccruedExpensesAndOtherCurrentLiabilities" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AccountsPayableCurrent" xlink:type="locator" xlink:label="us-gaap_AccountsPayableCurrent" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentNet" xlink:type="locator" xlink:label="us-gaap_PropertyPlantAndEquipmentNet" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PrepaidExpenseAndOtherAssetsCurrent" xlink:type="locator" xlink:label="us-gaap_PrepaidExpenseAndOtherAssetsCurrent" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ContractWithCustomerAssetNetCurrent" xlink:type="locator" xlink:label="us-gaap_ContractWithCustomerAssetNetCurrent" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AccountsReceivableNetCurrent" xlink:type="locator" xlink:label="us-gaap_AccountsReceivableNetCurrent" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CashAndCashEquivalentsAtCarryingValue" xlink:type="locator" xlink:label="us-gaap_CashAndCashEquivalentsAtCarryingValue" />
    <link:loc xlink:href="psn-20220331.xsd#psn_EmployeeStockOwnershipPlanMember" xlink:type="locator" xlink:label="psn_EmployeeStockOwnershipPlanMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PlanNameDomain" xlink:type="locator" xlink:label="us-gaap_PlanNameDomain" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PlanNameDomain" xlink:type="locator" xlink:label="us-gaap_PlanNameDomain_2" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PlanNameAxis" xlink:type="locator" xlink:label="us-gaap_PlanNameAxis" />
    <link:loc xlink:href="psn-20220331.xsd#psn_ConsolidatedJointVenturesMember" xlink:type="locator" xlink:label="psn_ConsolidatedJointVenturesMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RelatedPartyDomain" xlink:type="locator" xlink:label="us-gaap_RelatedPartyDomain" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RelatedPartyDomain" xlink:type="locator" xlink:label="us-gaap_RelatedPartyDomain_2" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:type="locator" xlink:label="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementTable" xlink:type="locator" xlink:label="us-gaap_StatementTable" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementLineItems" xlink:type="locator" xlink:label="us-gaap_StatementLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" order="10010.00" priority="2" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" order="10040.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:to="us-gaap_RelatedPartyDomain_2" order="10050.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:to="us-gaap_RelatedPartyDomain" order="10070.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RelatedPartyDomain" xlink:to="psn_ConsolidatedJointVenturesMember" order="10090.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_PlanNameAxis" order="10240.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_PlanNameAxis" xlink:to="us-gaap_PlanNameDomain_2" order="10250.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_PlanNameAxis" xlink:to="us-gaap_PlanNameDomain" order="10270.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PlanNameDomain" xlink:to="psn_EmployeeStockOwnershipPlanMember" order="10290.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_CashAndCashEquivalentsAtCarryingValue" order="10320.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_AccountsReceivableNetCurrent" order="10360.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ContractWithCustomerAssetNetCurrent" order="10400.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_PrepaidExpenseAndOtherAssetsCurrent" order="10440.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_PropertyPlantAndEquipmentNet" order="10480.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_AccountsPayableCurrent" order="10520.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="psn_AccruedExpensesAndOtherCurrentLiabilities" order="10560.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ContractWithCustomerLiabilityCurrent" order="10600.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_CommonStockParOrStatedValuePerShare" order="10640.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_CommonStockSharesAuthorized" order="10680.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_CommonStockSharesIssued" order="10720.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_CommonStockSharesOutstanding" order="10760.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_TreasuryStockShares" order="10840.00" priority="2" use="optional" />
  </link:definitionLink>
  <link:roleRef roleURI="http://www.parsons.com/20220331/taxonomy/role/StatementConsolidatedStatementsOfShareholderSEquityUnaudited" xlink:href="psn-20220331.xsd#Role_StatementConsolidatedStatementsOfShareholderSEquityUnaudited" xlink:type="simple" />
  <link:definitionLink xlink:type="extended" xlink:role="http://www.parsons.com/20220331/taxonomy/role/StatementConsolidatedStatementsOfShareholderSEquityUnaudited">
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue" xlink:type="locator" xlink:label="us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue" />
    <link:loc xlink:href="psn-20220331.xsd#psn_AdjustmentsToAdditionalPaidInCapitalRepurchasesOfCommonStock" xlink:type="locator" xlink:label="psn_AdjustmentsToAdditionalPaidInCapitalRepurchasesOfCommonStock" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockIssuedDuringPeriodValueNewIssues" xlink:type="locator" xlink:label="us-gaap_StockIssuedDuringPeriodValueNewIssues" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_MinorityInterestDecreaseFromDistributionsToNoncontrollingInterestHolders" xlink:type="locator" xlink:label="us-gaap_MinorityInterestDecreaseFromDistributionsToNoncontrollingInterestHolders" />
    <link:loc xlink:href="psn-20220331.xsd#psn_MinorityInterestIncreaseDecreaseFromContributionsToNoncontrollingInterestHolders" xlink:type="locator" xlink:label="psn_MinorityInterestIncreaseDecreaseFromContributionsToNoncontrollingInterestHolders" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax" xlink:type="locator" xlink:label="us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OtherComprehensiveIncomeForeignCurrencyTransactionAndTranslationGainLossArisingDuringPeriodNetOfTax" xlink:type="locator" xlink:label="us-gaap_OtherComprehensiveIncomeForeignCurrencyTransactionAndTranslationGainLossArisingDuringPeriodNetOfTax" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ProfitLoss" xlink:type="locator" xlink:label="us-gaap_ProfitLoss" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterestAbstract" xlink:type="locator" xlink:label="us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterestAbstract" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest" xlink:type="locator" xlink:label="us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest" />
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RevisionOfPriorPeriodAccountingStandardsUpdateAdjustmentMember" xlink:type="locator" xlink:label="srt_RevisionOfPriorPeriodAccountingStandardsUpdateAdjustmentMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RestatementDomain" xlink:type="locator" xlink:label="srt_RestatementDomain" />
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RestatementDomain" xlink:type="locator" xlink:label="srt_RestatementDomain_2" />
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RestatementAxis" xlink:type="locator" xlink:label="srt_RestatementAxis" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AccountingStandardsUpdate202006Member" xlink:type="locator" xlink:label="us-gaap_AccountingStandardsUpdate202006Member" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TypeOfAdoptionMember" xlink:type="locator" xlink:label="us-gaap_TypeOfAdoptionMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TypeOfAdoptionMember" xlink:type="locator" xlink:label="us-gaap_TypeOfAdoptionMember_2" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AdjustmentsForNewAccountingPronouncementsAxis" xlink:type="locator" xlink:label="us-gaap_AdjustmentsForNewAccountingPronouncementsAxis" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_NoncontrollingInterestMember" xlink:type="locator" xlink:label="us-gaap_NoncontrollingInterestMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ParentMember" xlink:type="locator" xlink:label="us-gaap_ParentMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember" xlink:type="locator" xlink:label="us-gaap_AccumulatedOtherComprehensiveIncomeMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetainedEarningsMember" xlink:type="locator" xlink:label="us-gaap_RetainedEarningsMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AdditionalPaidInCapitalMember" xlink:type="locator" xlink:label="us-gaap_AdditionalPaidInCapitalMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TreasuryStockMember" xlink:type="locator" xlink:label="us-gaap_TreasuryStockMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonStockMember" xlink:type="locator" xlink:label="us-gaap_CommonStockMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EquityComponentDomain" xlink:type="locator" xlink:label="us-gaap_EquityComponentDomain" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EquityComponentDomain" xlink:type="locator" xlink:label="us-gaap_EquityComponentDomain_2" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementEquityComponentsAxis" xlink:type="locator" xlink:label="us-gaap_StatementEquityComponentsAxis" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementTable" xlink:type="locator" xlink:label="us-gaap_StatementTable" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementLineItems" xlink:type="locator" xlink:label="us-gaap_StatementLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" order="10010.00" priority="2" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementEquityComponentsAxis" order="10040.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementEquityComponentsAxis" xlink:to="us-gaap_EquityComponentDomain_2" order="10050.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementEquityComponentsAxis" xlink:to="us-gaap_EquityComponentDomain" order="10070.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EquityComponentDomain" xlink:to="us-gaap_CommonStockMember" order="10090.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EquityComponentDomain" xlink:to="us-gaap_TreasuryStockMember" order="10110.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EquityComponentDomain" xlink:to="us-gaap_AdditionalPaidInCapitalMember" order="10130.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EquityComponentDomain" xlink:to="us-gaap_RetainedEarningsMember" order="10150.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EquityComponentDomain" xlink:to="us-gaap_AccumulatedOtherComprehensiveIncomeMember" order="10170.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EquityComponentDomain" xlink:to="us-gaap_ParentMember" order="10190.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EquityComponentDomain" xlink:to="us-gaap_NoncontrollingInterestMember" order="10210.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_AdjustmentsForNewAccountingPronouncementsAxis" order="10220.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_AdjustmentsForNewAccountingPronouncementsAxis" xlink:to="us-gaap_TypeOfAdoptionMember_2" order="10230.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_AdjustmentsForNewAccountingPronouncementsAxis" xlink:to="us-gaap_TypeOfAdoptionMember" order="10250.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TypeOfAdoptionMember" xlink:to="us-gaap_AccountingStandardsUpdate202006Member" order="10270.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="srt_RestatementAxis" order="10300.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="srt_RestatementAxis" xlink:to="srt_RestatementDomain_2" order="10310.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="srt_RestatementAxis" xlink:to="srt_RestatementDomain" order="10330.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_RestatementDomain" xlink:to="srt_RevisionOfPriorPeriodAccountingStandardsUpdateAdjustmentMember" order="10350.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest" order="10400.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterestAbstract" order="10760.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterestAbstract" xlink:to="us-gaap_ProfitLoss" order="11120.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterestAbstract" xlink:to="us-gaap_OtherComprehensiveIncomeForeignCurrencyTransactionAndTranslationGainLossArisingDuringPeriodNetOfTax" order="11300.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ComprehensiveIncomeNetOfTaxIncludingPortionAttributableToNoncontrollingInterestAbstract" xlink:to="us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax" order="11480.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="psn_MinorityInterestIncreaseDecreaseFromContributionsToNoncontrollingInterestHolders" order="11660.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_MinorityInterestDecreaseFromDistributionsToNoncontrollingInterestHolders" order="11840.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StockIssuedDuringPeriodValueNewIssues" order="12020.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="psn_AdjustmentsToAdditionalPaidInCapitalRepurchasesOfCommonStock" order="12200.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue" order="12380.00" priority="2" use="optional" />
  </link:definitionLink>
  <link:roleRef roleURI="http://www.parsons.com/20220331/taxonomy/role/DisclosureAcquisitionsTables" xlink:href="psn-20220331.xsd#Role_DisclosureAcquisitionsTables" xlink:type="simple" />
  <link:definitionLink xlink:type="extended" xlink:role="http://www.parsons.com/20220331/taxonomy/role/DisclosureAcquisitionsTables">
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionProFormaInformationTextBlock" xlink:type="locator" xlink:label="us-gaap_BusinessAcquisitionProFormaInformationTextBlock" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfFiniteLivedIntangibleAssetsAcquiredAsPartOfBusinessCombinationTextBlock" xlink:type="locator" xlink:label="us-gaap_ScheduleOfFiniteLivedIntangibleAssetsAcquiredAsPartOfBusinessCombinationTextBlock" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfRecognizedIdentifiedAssetsAcquiredAndLiabilitiesAssumedTableTextBlock" xlink:type="locator" xlink:label="us-gaap_ScheduleOfRecognizedIdentifiedAssetsAcquiredAndLiabilitiesAssumedTableTextBlock" />
    <link:loc xlink:href="psn-20220331.xsd#psn_BlackHorseSolutionsIncMember" xlink:type="locator" xlink:label="psn_BlackHorseSolutionsIncMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:type="locator" xlink:label="us-gaap_BusinessAcquisitionAcquireeDomain" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:type="locator" xlink:label="us-gaap_BusinessAcquisitionAcquireeDomain_2" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:type="locator" xlink:label="us-gaap_BusinessAcquisitionAxis" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementTable" xlink:type="locator" xlink:label="us-gaap_StatementTable" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementLineItems" xlink:type="locator" xlink:label="us-gaap_StatementLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" order="10010.00" priority="2" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_BusinessAcquisitionAxis" order="10040.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_BusinessAcquisitionAxis" xlink:to="us-gaap_BusinessAcquisitionAcquireeDomain_2" order="10050.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_BusinessAcquisitionAxis" xlink:to="us-gaap_BusinessAcquisitionAcquireeDomain" order="10070.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BusinessAcquisitionAcquireeDomain" xlink:to="psn_BlackHorseSolutionsIncMember" order="10090.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ScheduleOfRecognizedIdentifiedAssetsAcquiredAndLiabilitiesAssumedTableTextBlock" order="10240.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ScheduleOfFiniteLivedIntangibleAssetsAcquiredAsPartOfBusinessCombinationTextBlock" order="10290.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_BusinessAcquisitionProFormaInformationTextBlock" order="10340.00" priority="2" use="optional" />
  </link:definitionLink>
  <link:roleRef roleURI="http://www.parsons.com/20220331/taxonomy/role/DisclosureBasisOfPresentationAndPrinciplesOfConsolidationAdditionalInformationDetails" xlink:href="psn-20220331.xsd#Role_DisclosureBasisOfPresentationAndPrinciplesOfConsolidationAdditionalInformationDetails" xlink:type="simple" />
  <link:definitionLink xlink:type="extended" xlink:role="http://www.parsons.com/20220331/taxonomy/role/DisclosureBasisOfPresentationAndPrinciplesOfConsolidationAdditionalInformationDetails">
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockRepurchaseProgramAuthorizedAmount1" xlink:type="locator" xlink:label="us-gaap_StockRepurchaseProgramAuthorizedAmount1" />
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_MinimumMember" xlink:type="locator" xlink:label="srt_MinimumMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_MaximumMember" xlink:type="locator" xlink:label="srt_MaximumMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeMember" xlink:type="locator" xlink:label="srt_RangeMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeMember" xlink:type="locator" xlink:label="srt_RangeMember_2" />
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeAxis" xlink:type="locator" xlink:label="srt_RangeAxis" />
    <link:loc xlink:href="psn-20220331.xsd#psn_BasisOfPresentationAndPrinciplesOfConsolidationDisclosureTable" xlink:type="locator" xlink:label="psn_BasisOfPresentationAndPrinciplesOfConsolidationDisclosureTable" />
    <link:loc xlink:href="psn-20220331.xsd#psn_BasisOfPresentationAndPrinciplesOfConsolidationDisclosureLineItems" xlink:type="locator" xlink:label="psn_BasisOfPresentationAndPrinciplesOfConsolidationDisclosureLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="psn_BasisOfPresentationAndPrinciplesOfConsolidationDisclosureLineItems" xlink:to="psn_BasisOfPresentationAndPrinciplesOfConsolidationDisclosureTable" order="10010.00" priority="2" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="psn_BasisOfPresentationAndPrinciplesOfConsolidationDisclosureTable" xlink:to="srt_RangeAxis" order="10040.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="srt_RangeAxis" xlink:to="srt_RangeMember_2" order="10050.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="srt_RangeAxis" xlink:to="srt_RangeMember" order="10070.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_RangeMember" xlink:to="srt_MaximumMember" order="10090.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_RangeMember" xlink:to="srt_MinimumMember" order="10110.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="psn_BasisOfPresentationAndPrinciplesOfConsolidationDisclosureLineItems" xlink:to="us-gaap_StockRepurchaseProgramAuthorizedAmount1" order="10140.00" priority="2" use="optional" />
  </link:definitionLink>
  <link:roleRef roleURI="http://www.parsons.com/20220331/taxonomy/role/DisclosureNewAccountingPronouncementsAdditionalInformationDetails" xlink:href="psn-20220331.xsd#Role_DisclosureNewAccountingPronouncementsAdditionalInformationDetails" xlink:type="simple" />
  <link:definitionLink xlink:type="extended" xlink:role="http://www.parsons.com/20220331/taxonomy/role/DisclosureNewAccountingPronouncementsAdditionalInformationDetails">
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ChangeInAccountingPrincipleAccountingStandardsUpdateAdopted" xlink:type="locator" xlink:label="us-gaap_ChangeInAccountingPrincipleAccountingStandardsUpdateAdopted" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ChangeInAccountingPrincipleAccountingStandardsUpdateImmaterialEffect" xlink:type="locator" xlink:label="us-gaap_ChangeInAccountingPrincipleAccountingStandardsUpdateImmaterialEffect" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ChangeInAccountingPrincipleAccountingStandardsUpdateEarlyAdoption" xlink:type="locator" xlink:label="us-gaap_ChangeInAccountingPrincipleAccountingStandardsUpdateEarlyAdoption" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ChangeInAccountingPrincipleAccountingStandardsUpdateAdoptionDate" xlink:type="locator" xlink:label="us-gaap_ChangeInAccountingPrincipleAccountingStandardsUpdateAdoptionDate" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AccountingStandardsUpdate201912Member" xlink:type="locator" xlink:label="us-gaap_AccountingStandardsUpdate201912Member" />
    <link:loc xlink:href="psn-20220331.xsd#psn_AccountingStandardsUpdate202108Member" xlink:type="locator" xlink:label="psn_AccountingStandardsUpdate202108Member" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AccountingStandardsUpdate202006Member" xlink:type="locator" xlink:label="us-gaap_AccountingStandardsUpdate202006Member" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TypeOfAdoptionMember" xlink:type="locator" xlink:label="us-gaap_TypeOfAdoptionMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AdjustmentsForNewAccountingPronouncementsAxis" xlink:type="locator" xlink:label="us-gaap_AdjustmentsForNewAccountingPronouncementsAxis" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable" xlink:type="locator" xlink:label="us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems" xlink:type="locator" xlink:label="us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems" xlink:to="us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable" order="10010.00" priority="2" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable" xlink:to="us-gaap_AdjustmentsForNewAccountingPronouncementsAxis" order="10040.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_AdjustmentsForNewAccountingPronouncementsAxis" xlink:to="us-gaap_TypeOfAdoptionMember" order="10070.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TypeOfAdoptionMember" xlink:to="us-gaap_AccountingStandardsUpdate202006Member" order="10090.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TypeOfAdoptionMember" xlink:to="psn_AccountingStandardsUpdate202108Member" order="10150.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TypeOfAdoptionMember" xlink:to="us-gaap_AccountingStandardsUpdate201912Member" order="10250.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems" xlink:to="us-gaap_ChangeInAccountingPrincipleAccountingStandardsUpdateAdoptionDate" order="10320.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems" xlink:to="us-gaap_ChangeInAccountingPrincipleAccountingStandardsUpdateEarlyAdoption" order="10340.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems" xlink:to="us-gaap_ChangeInAccountingPrincipleAccountingStandardsUpdateImmaterialEffect" order="10360.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems" xlink:to="us-gaap_ChangeInAccountingPrincipleAccountingStandardsUpdateAdopted" order="10490.00" priority="2" use="optional" />
  </link:definitionLink>
  <link:roleRef roleURI="http://www.parsons.com/20220331/taxonomy/role/DisclosureAcquisitionsAdditionalInformationDetails" xlink:href="psn-20220331.xsd#Role_DisclosureAcquisitionsAdditionalInformationDetails" xlink:type="simple" />
  <link:definitionLink xlink:type="extended" xlink:role="http://www.parsons.com/20220331/taxonomy/role/DisclosureAcquisitionsAdditionalInformationDetails">
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_Goodwill" xlink:type="locator" xlink:label="us-gaap_Goodwill" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax" xlink:type="locator" xlink:label="us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionPurchasePriceAllocationGoodwillExpectedTaxDeductibleAmount" xlink:type="locator" xlink:label="us-gaap_BusinessAcquisitionPurchasePriceAllocationGoodwillExpectedTaxDeductibleAmount" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AmortizationOfIntangibleAssets" xlink:type="locator" xlink:label="us-gaap_AmortizationOfIntangibleAssets" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PaymentsToAcquireBusinessesGross" xlink:type="locator" xlink:label="us-gaap_PaymentsToAcquireBusinessesGross" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired" xlink:type="locator" xlink:label="us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired" />
    <link:loc xlink:href="psn-20220331.xsd#psn_EchoRidgeLLCMember" xlink:type="locator" xlink:label="psn_EchoRidgeLLCMember" />
    <link:loc xlink:href="psn-20220331.xsd#psn_BlackHorseSolutionsIncMember" xlink:type="locator" xlink:label="psn_BlackHorseSolutionsIncMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:type="locator" xlink:label="us-gaap_BusinessAcquisitionAcquireeDomain" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:type="locator" xlink:label="us-gaap_BusinessAcquisitionAxis" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" xlink:type="locator" xlink:label="us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:type="locator" xlink:label="us-gaap_BusinessAcquisitionLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" order="10010.00" priority="2" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" xlink:to="us-gaap_BusinessAcquisitionAxis" order="10040.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_BusinessAcquisitionAxis" xlink:to="us-gaap_BusinessAcquisitionAcquireeDomain" order="10070.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BusinessAcquisitionAcquireeDomain" xlink:to="psn_BlackHorseSolutionsIncMember" order="10090.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BusinessAcquisitionAcquireeDomain" xlink:to="psn_EchoRidgeLLCMember" order="10290.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired" order="10320.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="us-gaap_PaymentsToAcquireBusinessesGross" order="10340.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="us-gaap_AmortizationOfIntangibleAssets" order="10390.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="us-gaap_BusinessAcquisitionPurchasePriceAllocationGoodwillExpectedTaxDeductibleAmount" order="10430.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax" order="10500.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="us-gaap_Goodwill" order="10590.00" priority="2" use="optional" />
  </link:definitionLink>
  <link:roleRef roleURI="http://www.parsons.com/20220331/taxonomy/role/DisclosureAcquisitionsSummaryOfEstimatedFairValuesOfAssetsAcquiredAndLiabilitiesAssumedOnPreliminaryPurchasePriceAllocationDetails" xlink:href="psn-20220331.xsd#Role_DisclosureAcquisitionsSummaryOfEstimatedFairValuesOfAssetsAcquiredAndLiabilitiesAssumedOnPreliminaryPurchasePriceAllocationDetails" xlink:type="simple" />
  <link:definitionLink xlink:type="extended" xlink:role="http://www.parsons.com/20220331/taxonomy/role/DisclosureAcquisitionsSummaryOfEstimatedFairValuesOfAssetsAcquiredAndLiabilitiesAssumedOnPreliminaryPurchasePriceAllocationDetails">
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet" xlink:type="locator" xlink:label="us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesOther" xlink:type="locator" xlink:label="us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesOther" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxLiabilities" xlink:type="locator" xlink:label="us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxLiabilities" />
    <link:loc xlink:href="psn-20220331.xsd#psn_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOperatingLeasesLongTermLeaseLiabilities" xlink:type="locator" xlink:label="psn_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOperatingLeasesLongTermLeaseLiabilities" />
    <link:loc xlink:href="psn-20220331.xsd#psn_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOperatingLeasesShortTermLeaseLiabilities" xlink:type="locator" xlink:label="psn_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOperatingLeasesShortTermLeaseLiabilities" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ContractWithCustomerLiabilityCurrent" xlink:type="locator" xlink:label="us-gaap_ContractWithCustomerLiabilityCurrent" />
    <link:loc xlink:href="psn-20220331.xsd#psn_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesAccruedExpensesAndOtherLiabilities" xlink:type="locator" xlink:label="psn_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesAccruedExpensesAndOtherLiabilities" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesAccountsPayable" xlink:type="locator" xlink:label="us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesAccountsPayable" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill" xlink:type="locator" xlink:label="us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_Goodwill" xlink:type="locator" xlink:label="us-gaap_Goodwill" />
    <link:loc xlink:href="psn-20220331.xsd#psn_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOperatingLeasesRightOfUseAssets" xlink:type="locator" xlink:label="psn_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOperatingLeasesRightOfUseAssets" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment" xlink:type="locator" xlink:label="us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsPrepaidExpenseAndOtherAssets" xlink:type="locator" xlink:label="us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsPrepaidExpenseAndOtherAssets" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ContractWithCustomerAssetNetCurrent" xlink:type="locator" xlink:label="us-gaap_ContractWithCustomerAssetNetCurrent" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables" xlink:type="locator" xlink:label="us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents" xlink:type="locator" xlink:label="us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents" />
    <link:loc xlink:href="psn-20220331.xsd#psn_BlackHorseSolutionsIncMember" xlink:type="locator" xlink:label="psn_BlackHorseSolutionsIncMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:type="locator" xlink:label="us-gaap_BusinessAcquisitionAcquireeDomain" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:type="locator" xlink:label="us-gaap_BusinessAcquisitionAxis" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" xlink:type="locator" xlink:label="us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:type="locator" xlink:label="us-gaap_BusinessAcquisitionLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" order="10160.00" priority="2" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" xlink:to="us-gaap_BusinessAcquisitionAxis" order="10190.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_BusinessAcquisitionAxis" xlink:to="us-gaap_BusinessAcquisitionAcquireeDomain" order="10220.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BusinessAcquisitionAcquireeDomain" xlink:to="psn_BlackHorseSolutionsIncMember" order="10240.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents" order="10270.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables" order="10290.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="us-gaap_ContractWithCustomerAssetNetCurrent" order="10310.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsPrepaidExpenseAndOtherAssets" order="10330.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment" order="10350.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="psn_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOperatingLeasesRightOfUseAssets" order="10370.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="us-gaap_Goodwill" order="10390.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill" order="10410.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesAccountsPayable" order="10430.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="psn_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesAccruedExpensesAndOtherLiabilities" order="10450.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="us-gaap_ContractWithCustomerLiabilityCurrent" order="10470.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="psn_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOperatingLeasesShortTermLeaseLiabilities" order="10490.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="psn_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOperatingLeasesLongTermLeaseLiabilities" order="10510.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxLiabilities" order="10530.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesOther" order="10550.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet" order="10570.00" priority="2" use="optional" />
  </link:definitionLink>
  <link:roleRef roleURI="http://www.parsons.com/20220331/taxonomy/role/DisclosureAcquisitionsScheduleOfIntangibleAssetsValueOnPurchasePriceDetails" xlink:href="psn-20220331.xsd#Role_DisclosureAcquisitionsScheduleOfIntangibleAssetsValueOnPurchasePriceDetails" xlink:type="simple" />
  <link:definitionLink xlink:type="extended" xlink:role="http://www.parsons.com/20220331/taxonomy/role/DisclosureAcquisitionsScheduleOfIntangibleAssetsValueOnPurchasePriceDetails">
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife" xlink:type="locator" xlink:label="us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FinitelivedIntangibleAssetsAcquired1" xlink:type="locator" xlink:label="us-gaap_FinitelivedIntangibleAssetsAcquired1" />
    <link:loc xlink:href="psn-20220331.xsd#psn_BlackHorseSolutionsIncMember" xlink:type="locator" xlink:label="psn_BlackHorseSolutionsIncMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:type="locator" xlink:label="us-gaap_BusinessAcquisitionAcquireeDomain" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:type="locator" xlink:label="us-gaap_BusinessAcquisitionAxis" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DevelopedTechnologyRightsMember" xlink:type="locator" xlink:label="us-gaap_DevelopedTechnologyRightsMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_NoncompeteAgreementsMember" xlink:type="locator" xlink:label="us-gaap_NoncompeteAgreementsMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TradeNamesMember" xlink:type="locator" xlink:label="us-gaap_TradeNamesMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OrderOrProductionBacklogMember" xlink:type="locator" xlink:label="us-gaap_OrderOrProductionBacklogMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CustomerRelationshipsMember" xlink:type="locator" xlink:label="us-gaap_CustomerRelationshipsMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:type="locator" xlink:label="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:type="locator" xlink:label="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_2" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:type="locator" xlink:label="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsAcquiredAsPartOfBusinessCombinationTable" xlink:type="locator" xlink:label="us-gaap_FiniteLivedIntangibleAssetsAcquiredAsPartOfBusinessCombinationTable" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AcquiredFiniteLivedIntangibleAssetsLineItems" xlink:type="locator" xlink:label="us-gaap_AcquiredFiniteLivedIntangibleAssetsLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_AcquiredFiniteLivedIntangibleAssetsLineItems" xlink:to="us-gaap_FiniteLivedIntangibleAssetsAcquiredAsPartOfBusinessCombinationTable" order="10010.00" priority="2" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_FiniteLivedIntangibleAssetsAcquiredAsPartOfBusinessCombinationTable" xlink:to="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" order="10040.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:to="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_2" order="10050.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:to="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" order="10070.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:to="us-gaap_CustomerRelationshipsMember" order="10090.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:to="us-gaap_OrderOrProductionBacklogMember" order="10110.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:to="us-gaap_TradeNamesMember" order="10130.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:to="us-gaap_NoncompeteAgreementsMember" order="10150.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:to="us-gaap_DevelopedTechnologyRightsMember" order="10170.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_FiniteLivedIntangibleAssetsAcquiredAsPartOfBusinessCombinationTable" xlink:to="us-gaap_BusinessAcquisitionAxis" order="10180.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_BusinessAcquisitionAxis" xlink:to="us-gaap_BusinessAcquisitionAcquireeDomain" order="10210.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BusinessAcquisitionAcquireeDomain" xlink:to="psn_BlackHorseSolutionsIncMember" order="10230.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AcquiredFiniteLivedIntangibleAssetsLineItems" xlink:to="us-gaap_FinitelivedIntangibleAssetsAcquired1" order="10280.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AcquiredFiniteLivedIntangibleAssetsLineItems" xlink:to="us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife" order="10420.00" priority="2" use="optional" />
  </link:definitionLink>
  <link:roleRef roleURI="http://www.parsons.com/20220331/taxonomy/role/DisclosureAcquisitionsScheduleOfSupplementalProFormaInformationDetails" xlink:href="psn-20220331.xsd#Role_DisclosureAcquisitionsScheduleOfSupplementalProFormaInformationDetails" xlink:type="simple" />
  <link:definitionLink xlink:type="extended" xlink:role="http://www.parsons.com/20220331/taxonomy/role/DisclosureAcquisitionsScheduleOfSupplementalProFormaInformationDetails">
    <link:loc xlink:href="psn-20220331.xsd#psn_BusinessAcquisitionProFormaNetIncomeLossIncludingPortionAttributableToNoncontrollingInterest" xlink:type="locator" xlink:label="psn_BusinessAcquisitionProFormaNetIncomeLossIncludingPortionAttributableToNoncontrollingInterest" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionsProFormaRevenue" xlink:type="locator" xlink:label="us-gaap_BusinessAcquisitionsProFormaRevenue" />
    <link:loc xlink:href="psn-20220331.xsd#psn_BlackHorseSolutionsIncMember" xlink:type="locator" xlink:label="psn_BlackHorseSolutionsIncMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:type="locator" xlink:label="us-gaap_BusinessAcquisitionAcquireeDomain" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:type="locator" xlink:label="us-gaap_BusinessAcquisitionAxis" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" xlink:type="locator" xlink:label="us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:type="locator" xlink:label="us-gaap_BusinessAcquisitionLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" order="10010.00" priority="2" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" xlink:to="us-gaap_BusinessAcquisitionAxis" order="10040.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_BusinessAcquisitionAxis" xlink:to="us-gaap_BusinessAcquisitionAcquireeDomain" order="10070.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BusinessAcquisitionAcquireeDomain" xlink:to="psn_BlackHorseSolutionsIncMember" order="10090.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="us-gaap_BusinessAcquisitionsProFormaRevenue" order="10140.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BusinessAcquisitionLineItems" xlink:to="psn_BusinessAcquisitionProFormaNetIncomeLossIncludingPortionAttributableToNoncontrollingInterest" order="10180.00" priority="2" use="optional" />
  </link:definitionLink>
  <link:roleRef roleURI="http://www.parsons.com/20220331/taxonomy/role/DisclosureContractsWithCustomersSummaryOfDisaggregationOfRevenueDetails" xlink:href="psn-20220331.xsd#Role_DisclosureContractsWithCustomersSummaryOfDisaggregationOfRevenueDetails" xlink:type="simple" />
  <link:definitionLink xlink:type="extended" xlink:role="http://www.parsons.com/20220331/taxonomy/role/DisclosureContractsWithCustomersSummaryOfDisaggregationOfRevenueDetails">
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax" xlink:type="locator" xlink:label="us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax" />
    <link:loc xlink:href="psn-20220331.xsd#psn_CostPlusMember" xlink:type="locator" xlink:label="psn_CostPlusMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TimeAndMaterialsContractMember" xlink:type="locator" xlink:label="us-gaap_TimeAndMaterialsContractMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FixedPriceContractMember" xlink:type="locator" xlink:label="us-gaap_FixedPriceContractMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ContractWithCustomerBasisOfPricingDomain" xlink:type="locator" xlink:label="us-gaap_ContractWithCustomerBasisOfPricingDomain" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ContractWithCustomerBasisOfPricingDomain" xlink:type="locator" xlink:label="us-gaap_ContractWithCustomerBasisOfPricingDomain_2" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ContractWithCustomerBasisOfPricingAxis" xlink:type="locator" xlink:label="us-gaap_ContractWithCustomerBasisOfPricingAxis" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DisaggregationOfRevenueTable" xlink:type="locator" xlink:label="us-gaap_DisaggregationOfRevenueTable" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DisaggregationOfRevenueLineItems" xlink:type="locator" xlink:label="us-gaap_DisaggregationOfRevenueLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_DisaggregationOfRevenueLineItems" xlink:to="us-gaap_DisaggregationOfRevenueTable" order="10010.00" priority="2" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_DisaggregationOfRevenueTable" xlink:to="us-gaap_ContractWithCustomerBasisOfPricingAxis" order="10040.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_ContractWithCustomerBasisOfPricingAxis" xlink:to="us-gaap_ContractWithCustomerBasisOfPricingDomain_2" order="10050.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_ContractWithCustomerBasisOfPricingAxis" xlink:to="us-gaap_ContractWithCustomerBasisOfPricingDomain" order="10070.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ContractWithCustomerBasisOfPricingDomain" xlink:to="us-gaap_FixedPriceContractMember" order="10090.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ContractWithCustomerBasisOfPricingDomain" xlink:to="us-gaap_TimeAndMaterialsContractMember" order="10110.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ContractWithCustomerBasisOfPricingDomain" xlink:to="psn_CostPlusMember" order="10130.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DisaggregationOfRevenueLineItems" xlink:to="us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax" order="10160.00" priority="2" use="optional" />
  </link:definitionLink>
  <link:roleRef roleURI="http://www.parsons.com/20220331/taxonomy/role/DisclosureContractsWithCustomersSummaryOfRemainingUnsatisfiedPerformanceObligationsExpectToSatisfyDetails" xlink:href="psn-20220331.xsd#Role_DisclosureContractsWithCustomersSummaryOfRemainingUnsatisfiedPerformanceObligationsExpectToSatisfyDetails" xlink:type="simple" />
  <link:definitionLink xlink:type="extended" xlink:role="http://www.parsons.com/20220331/taxonomy/role/DisclosureContractsWithCustomersSummaryOfRemainingUnsatisfiedPerformanceObligationsExpectToSatisfyDetails">
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1" xlink:type="locator" xlink:label="us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenueRemainingPerformanceObligation" xlink:type="locator" xlink:label="us-gaap_RevenueRemainingPerformanceObligation" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis" xlink:type="locator" xlink:label="us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis" />
    <link:loc xlink:href="psn-20220331.xsd#psn_CriticalInfrastructureSegmentMember" xlink:type="locator" xlink:label="psn_CriticalInfrastructureSegmentMember" />
    <link:loc xlink:href="psn-20220331.xsd#psn_FederalSolutionSegmentMember" xlink:type="locator" xlink:label="psn_FederalSolutionSegmentMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SegmentDomain" xlink:type="locator" xlink:label="us-gaap_SegmentDomain" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SegmentDomain" xlink:type="locator" xlink:label="us-gaap_SegmentDomain_2" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementBusinessSegmentsAxis" xlink:type="locator" xlink:label="us-gaap_StatementBusinessSegmentsAxis" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable" xlink:type="locator" xlink:label="us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems" xlink:type="locator" xlink:label="us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems" xlink:to="us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable" order="10010.00" priority="2" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable" xlink:to="us-gaap_StatementBusinessSegmentsAxis" order="10040.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementBusinessSegmentsAxis" xlink:to="us-gaap_SegmentDomain_2" order="10050.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementBusinessSegmentsAxis" xlink:to="us-gaap_SegmentDomain" order="10070.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SegmentDomain" xlink:to="psn_FederalSolutionSegmentMember" order="10090.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SegmentDomain" xlink:to="psn_CriticalInfrastructureSegmentMember" order="10110.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable" xlink:to="us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis" order="10120.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems" xlink:to="us-gaap_RevenueRemainingPerformanceObligation" order="10150.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems" xlink:to="us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1" order="10230.00" priority="2" use="optional" />
  </link:definitionLink>
  <link:roleRef roleURI="http://www.parsons.com/20220331/taxonomy/role/DisclosureContractsWithCustomersSummaryOfRemainingUnsatisfiedPerformanceObligationsExpectToSatisfyDetailsDefault" xlink:href="psn-20220331.xsd#Role_DisclosureContractsWithCustomersSummaryOfRemainingUnsatisfiedPerformanceObligationsExpectToSatisfyDetailsDefault" xlink:type="simple" />
  <link:definitionLink xlink:type="extended" xlink:role="http://www.parsons.com/20220331/taxonomy/role/DisclosureContractsWithCustomersSummaryOfRemainingUnsatisfiedPerformanceObligationsExpectToSatisfyDetailsDefault">
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1" xlink:type="locator" xlink:label="us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenueRemainingPerformanceObligation" xlink:type="locator" xlink:label="us-gaap_RevenueRemainingPerformanceObligation" />
    <link:loc xlink:href="psn-20220331.xsd#psn_CriticalInfrastructureSegmentMember" xlink:type="locator" xlink:label="psn_CriticalInfrastructureSegmentMember" />
    <link:loc xlink:href="psn-20220331.xsd#psn_FederalSolutionSegmentMember" xlink:type="locator" xlink:label="psn_FederalSolutionSegmentMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SegmentDomain" xlink:type="locator" xlink:label="us-gaap_SegmentDomain" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementBusinessSegmentsAxis" xlink:type="locator" xlink:label="us-gaap_StatementBusinessSegmentsAxis" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable" xlink:type="locator" xlink:label="us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems" xlink:type="locator" xlink:label="us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems" xlink:to="us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable" order="10010.00" priority="2" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable" xlink:to="us-gaap_StatementBusinessSegmentsAxis" order="10020.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementBusinessSegmentsAxis" xlink:to="us-gaap_SegmentDomain" order="10030.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SegmentDomain" xlink:to="psn_FederalSolutionSegmentMember" order="10040.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SegmentDomain" xlink:to="psn_CriticalInfrastructureSegmentMember" order="10050.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems" xlink:to="us-gaap_RevenueRemainingPerformanceObligation" order="10060.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems" xlink:to="us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1" order="10100.00" priority="2" use="optional" />
  </link:definitionLink>
  <link:roleRef roleURI="http://www.parsons.com/20220331/taxonomy/role/DisclosureLeasesAdditionalInformationDetails" xlink:href="psn-20220331.xsd#Role_DisclosureLeasesAdditionalInformationDetails" xlink:type="simple" />
  <link:definitionLink xlink:type="extended" xlink:role="http://www.parsons.com/20220331/taxonomy/role/DisclosureLeasesAdditionalInformationDetails">
    <link:loc xlink:href="psn-20220331.xsd#psn_OperatingLeaseNotYetCommencedExpense" xlink:type="locator" xlink:label="psn_OperatingLeaseNotYetCommencedExpense" />
    <link:loc xlink:href="psn-20220331.xsd#psn_LesseeLeaseExistenceOfOptionToTerminate" xlink:type="locator" xlink:label="psn_LesseeLeaseExistenceOfOptionToTerminate" />
    <link:loc xlink:href="psn-20220331.xsd#psn_LesseeLeaseOptionToTerminate" xlink:type="locator" xlink:label="psn_LesseeLeaseOptionToTerminate" />
    <link:loc xlink:href="psn-20220331.xsd#psn_LesseeLeaseExistenceOfOptionToExtend" xlink:type="locator" xlink:label="psn_LesseeLeaseExistenceOfOptionToExtend" />
    <link:loc xlink:href="psn-20220331.xsd#psn_LesseeLeaseOptionToExtend" xlink:type="locator" xlink:label="psn_LesseeLeaseOptionToExtend" />
    <link:loc xlink:href="psn-20220331.xsd#psn_LesseeLeaseTermOfContract" xlink:type="locator" xlink:label="psn_LesseeLeaseTermOfContract" />
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_MaximumMember" xlink:type="locator" xlink:label="srt_MaximumMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_MinimumMember" xlink:type="locator" xlink:label="srt_MinimumMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeMember" xlink:type="locator" xlink:label="srt_RangeMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeAxis" xlink:type="locator" xlink:label="srt_RangeAxis" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LesseeLeaseDescriptionTable" xlink:type="locator" xlink:label="us-gaap_LesseeLeaseDescriptionTable" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LesseeLeaseDescriptionLineItems" xlink:type="locator" xlink:label="us-gaap_LesseeLeaseDescriptionLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_LesseeLeaseDescriptionLineItems" xlink:to="us-gaap_LesseeLeaseDescriptionTable" order="10010.00" priority="2" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_LesseeLeaseDescriptionTable" xlink:to="srt_RangeAxis" order="10040.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="srt_RangeAxis" xlink:to="srt_RangeMember" order="10070.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_RangeMember" xlink:to="srt_MinimumMember" order="10090.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_RangeMember" xlink:to="srt_MaximumMember" order="10110.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LesseeLeaseDescriptionLineItems" xlink:to="psn_LesseeLeaseTermOfContract" order="10140.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LesseeLeaseDescriptionLineItems" xlink:to="psn_LesseeLeaseOptionToExtend" order="10220.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LesseeLeaseDescriptionLineItems" xlink:to="psn_LesseeLeaseExistenceOfOptionToExtend" order="10260.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LesseeLeaseDescriptionLineItems" xlink:to="psn_LesseeLeaseOptionToTerminate" order="10300.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LesseeLeaseDescriptionLineItems" xlink:to="psn_LesseeLeaseExistenceOfOptionToTerminate" order="10340.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LesseeLeaseDescriptionLineItems" xlink:to="psn_OperatingLeaseNotYetCommencedExpense" order="10410.00" priority="2" use="optional" />
  </link:definitionLink>
  <link:roleRef roleURI="http://fasb.org/us-gaap/role/eedm/ExtensibleEnumerationLists" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-roles-2021-01-31.xsd#eedm" xlink:type="simple" />
  <link:definitionLink xlink:type="extended" xlink:role="http://fasb.org/us-gaap/role/eedm/ExtensibleEnumerationLists">
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OtherLiabilitiesNoncurrent" xlink:type="locator" xlink:label="us-gaap_OtherLiabilitiesNoncurrent" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LiabilitiesNoncurrentAbstract" xlink:type="locator" xlink:label="us-gaap_LiabilitiesNoncurrentAbstract" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OtherLiabilitiesCurrent" xlink:type="locator" xlink:label="us-gaap_OtherLiabilitiesCurrent" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LiabilitiesCurrentAbstract" xlink:type="locator" xlink:label="us-gaap_LiabilitiesCurrentAbstract" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LiabilitiesCurrentAbstract" xlink:to="us-gaap_OtherLiabilitiesCurrent" order="10530.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LiabilitiesNoncurrentAbstract" xlink:to="us-gaap_OtherLiabilitiesNoncurrent" order="10630.00" priority="2" use="optional" />
  </link:definitionLink>
  <link:roleRef roleURI="http://www.parsons.com/20220331/taxonomy/role/DisclosureGoodwillSummaryOfChangesInCarryingValueOfGoodwillByReportingSegmentDetails" xlink:href="psn-20220331.xsd#Role_DisclosureGoodwillSummaryOfChangesInCarryingValueOfGoodwillByReportingSegmentDetails" xlink:type="simple" />
  <link:definitionLink xlink:type="extended" xlink:role="http://www.parsons.com/20220331/taxonomy/role/DisclosureGoodwillSummaryOfChangesInCarryingValueOfGoodwillByReportingSegmentDetails">
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_GoodwillForeignCurrencyTranslationGainLoss" xlink:type="locator" xlink:label="us-gaap_GoodwillForeignCurrencyTranslationGainLoss" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_GoodwillAcquiredDuringPeriod" xlink:type="locator" xlink:label="us-gaap_GoodwillAcquiredDuringPeriod" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_Goodwill" xlink:type="locator" xlink:label="us-gaap_Goodwill" />
    <link:loc xlink:href="psn-20220331.xsd#psn_CriticalInfrastructureSegmentMember" xlink:type="locator" xlink:label="psn_CriticalInfrastructureSegmentMember" />
    <link:loc xlink:href="psn-20220331.xsd#psn_FederalSolutionSegmentMember" xlink:type="locator" xlink:label="psn_FederalSolutionSegmentMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SegmentDomain" xlink:type="locator" xlink:label="us-gaap_SegmentDomain" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementBusinessSegmentsAxis" xlink:type="locator" xlink:label="us-gaap_StatementBusinessSegmentsAxis" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfGoodwillTable" xlink:type="locator" xlink:label="us-gaap_ScheduleOfGoodwillTable" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_GoodwillLineItems" xlink:type="locator" xlink:label="us-gaap_GoodwillLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_GoodwillLineItems" xlink:to="us-gaap_ScheduleOfGoodwillTable" order="10010.00" priority="2" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfGoodwillTable" xlink:to="us-gaap_StatementBusinessSegmentsAxis" order="10040.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementBusinessSegmentsAxis" xlink:to="us-gaap_SegmentDomain" order="10070.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SegmentDomain" xlink:to="psn_FederalSolutionSegmentMember" order="10090.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SegmentDomain" xlink:to="psn_CriticalInfrastructureSegmentMember" order="10110.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_GoodwillLineItems" xlink:to="us-gaap_Goodwill" order="10140.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_GoodwillLineItems" xlink:to="us-gaap_GoodwillAcquiredDuringPeriod" order="10220.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_GoodwillLineItems" xlink:to="us-gaap_GoodwillForeignCurrencyTranslationGainLoss" order="10300.00" priority="2" use="optional" />
  </link:definitionLink>
  <link:roleRef roleURI="http://www.parsons.com/20220331/taxonomy/role/DisclosureIntangibleAssetsScheduleOfGrossAmountAndAccumulatedAmortizationOfIntangibleAssetsDetails" xlink:href="psn-20220331.xsd#Role_DisclosureIntangibleAssetsScheduleOfGrossAmountAndAccumulatedAmortizationOfIntangibleAssetsDetails" xlink:type="simple" />
  <link:definitionLink xlink:type="extended" xlink:role="http://www.parsons.com/20220331/taxonomy/role/DisclosureIntangibleAssetsScheduleOfGrossAmountAndAccumulatedAmortizationOfIntangibleAssetsDetails">
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife" xlink:type="locator" xlink:label="us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsNet" xlink:type="locator" xlink:label="us-gaap_FiniteLivedIntangibleAssetsNet" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization" xlink:type="locator" xlink:label="us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsGross" xlink:type="locator" xlink:label="us-gaap_FiniteLivedIntangibleAssetsGross" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OtherIntangibleAssetsMember" xlink:type="locator" xlink:label="us-gaap_OtherIntangibleAssetsMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_InProcessResearchAndDevelopmentMember" xlink:type="locator" xlink:label="us-gaap_InProcessResearchAndDevelopmentMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_NoncompeteAgreementsMember" xlink:type="locator" xlink:label="us-gaap_NoncompeteAgreementsMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TradeNamesMember" xlink:type="locator" xlink:label="us-gaap_TradeNamesMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DevelopedTechnologyRightsMember" xlink:type="locator" xlink:label="us-gaap_DevelopedTechnologyRightsMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LeaseAgreementsMember" xlink:type="locator" xlink:label="us-gaap_LeaseAgreementsMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CustomerRelationshipsMember" xlink:type="locator" xlink:label="us-gaap_CustomerRelationshipsMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OrderOrProductionBacklogMember" xlink:type="locator" xlink:label="us-gaap_OrderOrProductionBacklogMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:type="locator" xlink:label="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:type="locator" xlink:label="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfAcquiredFiniteLivedIntangibleAssetByMajorClassTable" xlink:type="locator" xlink:label="us-gaap_ScheduleOfAcquiredFiniteLivedIntangibleAssetByMajorClassTable" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AcquiredFiniteLivedIntangibleAssetsLineItems" xlink:type="locator" xlink:label="us-gaap_AcquiredFiniteLivedIntangibleAssetsLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_AcquiredFiniteLivedIntangibleAssetsLineItems" xlink:to="us-gaap_ScheduleOfAcquiredFiniteLivedIntangibleAssetByMajorClassTable" order="10030.00" priority="2" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfAcquiredFiniteLivedIntangibleAssetByMajorClassTable" xlink:to="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" order="10060.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:to="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" order="10090.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:to="us-gaap_OrderOrProductionBacklogMember" order="10110.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:to="us-gaap_CustomerRelationshipsMember" order="10130.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:to="us-gaap_LeaseAgreementsMember" order="10150.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:to="us-gaap_DevelopedTechnologyRightsMember" order="10170.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:to="us-gaap_TradeNamesMember" order="10190.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:to="us-gaap_NoncompeteAgreementsMember" order="10210.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:to="us-gaap_InProcessResearchAndDevelopmentMember" order="10230.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:to="us-gaap_OtherIntangibleAssetsMember" order="10250.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AcquiredFiniteLivedIntangibleAssetsLineItems" xlink:to="us-gaap_FiniteLivedIntangibleAssetsGross" order="10480.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AcquiredFiniteLivedIntangibleAssetsLineItems" xlink:to="us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization" order="10700.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AcquiredFiniteLivedIntangibleAssetsLineItems" xlink:to="us-gaap_FiniteLivedIntangibleAssetsNet" order="10920.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AcquiredFiniteLivedIntangibleAssetsLineItems" xlink:to="us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife" order="11830.00" priority="2" use="optional" />
  </link:definitionLink>
  <link:roleRef roleURI="http://www.parsons.com/20220331/taxonomy/role/DisclosurePropertyAndEquipmentNetScheduleOfPropertyAndEquipmentNetDetails" xlink:href="psn-20220331.xsd#Role_DisclosurePropertyAndEquipmentNetScheduleOfPropertyAndEquipmentNetDetails" xlink:type="simple" />
  <link:definitionLink xlink:type="extended" xlink:role="http://www.parsons.com/20220331/taxonomy/role/DisclosurePropertyAndEquipmentNetScheduleOfPropertyAndEquipmentNetDetails">
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentUsefulLife" xlink:type="locator" xlink:label="us-gaap_PropertyPlantAndEquipmentUsefulLife" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentNet" xlink:type="locator" xlink:label="us-gaap_PropertyPlantAndEquipmentNet" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment" xlink:type="locator" xlink:label="us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentGross" xlink:type="locator" xlink:label="us-gaap_PropertyPlantAndEquipmentGross" />
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_MaximumMember" xlink:type="locator" xlink:label="srt_MaximumMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_MinimumMember" xlink:type="locator" xlink:label="srt_MinimumMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeMember" xlink:type="locator" xlink:label="srt_RangeMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeAxis" xlink:type="locator" xlink:label="srt_RangeAxis" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ConstructionInProgressMember" xlink:type="locator" xlink:label="us-gaap_ConstructionInProgressMember" />
    <link:loc xlink:href="psn-20220331.xsd#psn_ConstructionEquipmentMember" xlink:type="locator" xlink:label="psn_ConstructionEquipmentMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ComputerEquipmentMember" xlink:type="locator" xlink:label="us-gaap_ComputerEquipmentMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FurnitureAndFixturesMember" xlink:type="locator" xlink:label="us-gaap_FurnitureAndFixturesMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BuildingAndBuildingImprovementsMember" xlink:type="locator" xlink:label="us-gaap_BuildingAndBuildingImprovementsMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:type="locator" xlink:label="us-gaap_PropertyPlantAndEquipmentTypeDomain" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:type="locator" xlink:label="us-gaap_PropertyPlantAndEquipmentTypeDomain_2" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:type="locator" xlink:label="us-gaap_PropertyPlantAndEquipmentByTypeAxis" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:type="locator" xlink:label="us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentLineItems" xlink:type="locator" xlink:label="us-gaap_PropertyPlantAndEquipmentLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_PropertyPlantAndEquipmentLineItems" xlink:to="us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" order="10030.00" priority="2" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:to="us-gaap_PropertyPlantAndEquipmentByTypeAxis" order="10060.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:to="us-gaap_PropertyPlantAndEquipmentTypeDomain_2" order="10070.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:to="us-gaap_PropertyPlantAndEquipmentTypeDomain" order="10090.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="us-gaap_BuildingAndBuildingImprovementsMember" order="10110.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="us-gaap_FurnitureAndFixturesMember" order="10130.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="us-gaap_ComputerEquipmentMember" order="10150.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="psn_ConstructionEquipmentMember" order="10170.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="us-gaap_ConstructionInProgressMember" order="10190.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:to="srt_RangeAxis" order="10340.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="srt_RangeAxis" xlink:to="srt_RangeMember" order="10370.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_RangeMember" xlink:to="srt_MinimumMember" order="10390.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_RangeMember" xlink:to="srt_MaximumMember" order="10410.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentLineItems" xlink:to="us-gaap_PropertyPlantAndEquipmentGross" order="10440.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentLineItems" xlink:to="us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment" order="10680.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentLineItems" xlink:to="us-gaap_PropertyPlantAndEquipmentNet" order="10720.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentLineItems" xlink:to="us-gaap_PropertyPlantAndEquipmentUsefulLife" order="10790.00" priority="2" use="optional" />
  </link:definitionLink>
  <link:roleRef roleURI="http://www.parsons.com/20220331/taxonomy/role/DisclosureDebtAndCreditFacilitiesScheduleOfDebtDetails" xlink:href="psn-20220331.xsd#Role_DisclosureDebtAndCreditFacilitiesScheduleOfDebtDetails" xlink:type="simple" />
  <link:definitionLink xlink:type="extended" xlink:role="http://www.parsons.com/20220331/taxonomy/role/DisclosureDebtAndCreditFacilitiesScheduleOfDebtDetails">
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtLongtermAndShorttermCombinedAmount" xlink:type="locator" xlink:label="us-gaap_DebtLongtermAndShorttermCombinedAmount" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LongTermDebt" xlink:type="locator" xlink:label="us-gaap_LongTermDebt" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DeferredFinanceCostsNoncurrentNet" xlink:type="locator" xlink:label="us-gaap_DeferredFinanceCostsNoncurrentNet" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ConvertibleNotesPayable" xlink:type="locator" xlink:label="us-gaap_ConvertibleNotesPayable" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentCarryingAmount" xlink:type="locator" xlink:label="us-gaap_DebtInstrumentCarryingAmount" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SeniorNotesMember" xlink:type="locator" xlink:label="us-gaap_SeniorNotesMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LongtermDebtTypeDomain" xlink:type="locator" xlink:label="us-gaap_LongtermDebtTypeDomain" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LongtermDebtTypeDomain" xlink:type="locator" xlink:label="us-gaap_LongtermDebtTypeDomain_2" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LongtermDebtTypeAxis" xlink:type="locator" xlink:label="us-gaap_LongtermDebtTypeAxis" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentTable" xlink:type="locator" xlink:label="us-gaap_DebtInstrumentTable" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentLineItems" xlink:type="locator" xlink:label="us-gaap_DebtInstrumentLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_DebtInstrumentTable" order="10050.00" priority="2" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_DebtInstrumentTable" xlink:to="us-gaap_LongtermDebtTypeAxis" order="10080.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_LongtermDebtTypeAxis" xlink:to="us-gaap_LongtermDebtTypeDomain_2" order="10090.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_LongtermDebtTypeAxis" xlink:to="us-gaap_LongtermDebtTypeDomain" order="10110.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LongtermDebtTypeDomain" xlink:to="us-gaap_SeniorNotesMember" order="10130.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_DebtInstrumentCarryingAmount" order="10160.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_ConvertibleNotesPayable" order="10200.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_DeferredFinanceCostsNoncurrentNet" order="10240.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_LongTermDebt" order="10280.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_DebtLongtermAndShorttermCombinedAmount" order="10320.00" priority="2" use="optional" />
  </link:definitionLink>
  <link:roleRef roleURI="http://www.parsons.com/20220331/taxonomy/role/DisclosureDebtAndCreditFacilitiesAdditionalInformationDetails" xlink:href="psn-20220331.xsd#Role_DisclosureDebtAndCreditFacilitiesAdditionalInformationDetails" xlink:type="simple" />
  <link:definitionLink xlink:type="extended" xlink:role="http://www.parsons.com/20220331/taxonomy/role/DisclosureDebtAndCreditFacilitiesAdditionalInformationDetails">
    <link:loc xlink:href="psn-20220331.xsd#psn_AdjustmentToDeferredTaxAssetThroughRetainedEarnings" xlink:type="locator" xlink:label="psn_AdjustmentToDeferredTaxAssetThroughRetainedEarnings" />
    <link:loc xlink:href="psn-20220331.xsd#psn_AdditionalDeferredTaxLiabilityRelatedToCapitalizedDebtIssuanceCost" xlink:type="locator" xlink:label="psn_AdditionalDeferredTaxLiabilityRelatedToCapitalizedDebtIssuanceCost" />
    <link:loc xlink:href="psn-20220331.xsd#psn_ReversalOfDeferredTaxLiability" xlink:type="locator" xlink:label="psn_ReversalOfDeferredTaxLiability" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DeferredIncomeTaxAssetsNet" xlink:type="locator" xlink:label="us-gaap_DeferredIncomeTaxAssetsNet" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DeferredIncomeTaxLiabilitiesNet" xlink:type="locator" xlink:label="us-gaap_DeferredIncomeTaxLiabilitiesNet" />
    <link:loc xlink:href="psn-20220331.xsd#psn_NetCostOfPurchaseOfBondHedgesAndSaleOfWarrants" xlink:type="locator" xlink:label="psn_NetCostOfPurchaseOfBondHedgesAndSaleOfWarrants" />
    <link:loc xlink:href="psn-20220331.xsd#psn_PercentageOfBondHedgeAndWarrantTransactionsIncreasedConversionPrice" xlink:type="locator" xlink:label="psn_PercentageOfBondHedgeAndWarrantTransactionsIncreasedConversionPrice" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ProceedsFromIssuanceOfWarrants" xlink:type="locator" xlink:label="us-gaap_ProceedsFromIssuanceOfWarrants" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1" xlink:type="locator" xlink:label="us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1" />
    <link:loc xlink:href="psn-20220331.xsd#psn_NumberOfCommonStockAcquiredByOffsettingSaleWarrants" xlink:type="locator" xlink:label="psn_NumberOfCommonStockAcquiredByOffsettingSaleWarrants" />
    <link:loc xlink:href="psn-20220331.xsd#psn_CostOfConvertibleNoteHedgeTransaction" xlink:type="locator" xlink:label="psn_CostOfConvertibleNoteHedgeTransaction" />
    <link:loc xlink:href="psn-20220331.xsd#psn_DebtInstrumentNumberOfSharesIssuableUponConversion" xlink:type="locator" xlink:label="psn_DebtInstrumentNumberOfSharesIssuableUponConversion" />
    <link:loc xlink:href="psn-20220331.xsd#psn_BondHedgeTerm" xlink:type="locator" xlink:label="psn_BondHedgeTerm" />
    <link:loc xlink:href="psn-20220331.xsd#psn_CarryingAmountOfConvertibleSeniorNotesAdjustedInRetainedEarnings" xlink:type="locator" xlink:label="psn_CarryingAmountOfConvertibleSeniorNotesAdjustedInRetainedEarnings" />
    <link:loc xlink:href="psn-20220331.xsd#psn_ReclassificationOfEquityComponentOfDebtIssuanceCostsToLiabilities" xlink:type="locator" xlink:label="psn_ReclassificationOfEquityComponentOfDebtIssuanceCostsToLiabilities" />
    <link:loc xlink:href="psn-20220331.xsd#psn_ReclassificationOfEquityComponentOfConvertibleSeniorNotesToLiabilities" xlink:type="locator" xlink:label="psn_ReclassificationOfEquityComponentOfConvertibleSeniorNotesToLiabilities" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ConvertibleDebt" xlink:type="locator" xlink:label="us-gaap_ConvertibleDebt" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentInterestRateEffectivePercentage" xlink:type="locator" xlink:label="us-gaap_DebtInstrumentInterestRateEffectivePercentage" />
    <link:loc xlink:href="psn-20220331.xsd#psn_DebtInstrumentConvertibleNotesAdditionalPaidInCapital" xlink:type="locator" xlink:label="psn_DebtInstrumentConvertibleNotesAdditionalPaidInCapital" />
    <link:loc xlink:href="psn-20220331.xsd#psn_DebtInstrumentConvertibleDebtLiability" xlink:type="locator" xlink:label="psn_DebtInstrumentConvertibleDebtLiability" />
    <link:loc xlink:href="psn-20220331.xsd#psn_PercentageOfConvertibleSeniorNotesRepurchasedAtCash" xlink:type="locator" xlink:label="psn_PercentageOfConvertibleSeniorNotesRepurchasedAtCash" />
    <link:loc xlink:href="psn-20220331.xsd#psn_PercentageOfPrincipalAmountToRedeemConvertibleSeniorNotes" xlink:type="locator" xlink:label="psn_PercentageOfPrincipalAmountToRedeemConvertibleSeniorNotes" />
    <link:loc xlink:href="psn-20220331.xsd#psn_DebtInstrumentConvertibleMeasurementPeriodPercentage" xlink:type="locator" xlink:label="psn_DebtInstrumentConvertibleMeasurementPeriodPercentage" />
    <link:loc xlink:href="psn-20220331.xsd#psn_DebtInstrumentConvertiblePrincipalAmount" xlink:type="locator" xlink:label="psn_DebtInstrumentConvertiblePrincipalAmount" />
    <link:loc xlink:href="psn-20220331.xsd#psn_DebtInstrumentConvertibleConsecutiveBusinessDayPeriod" xlink:type="locator" xlink:label="psn_DebtInstrumentConvertibleConsecutiveBusinessDayPeriod" />
    <link:loc xlink:href="psn-20220331.xsd#psn_DebtInstrumentConvertibleBusinessPeriod" xlink:type="locator" xlink:label="psn_DebtInstrumentConvertibleBusinessPeriod" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentConvertibleThresholdPercentageOfStockPriceTrigger" xlink:type="locator" xlink:label="us-gaap_DebtInstrumentConvertibleThresholdPercentageOfStockPriceTrigger" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentConvertibleThresholdConsecutiveTradingDays1" xlink:type="locator" xlink:label="us-gaap_DebtInstrumentConvertibleThresholdConsecutiveTradingDays1" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentConvertibleThresholdTradingDays" xlink:type="locator" xlink:label="us-gaap_DebtInstrumentConvertibleThresholdTradingDays" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentConvertibleConversionPrice1" xlink:type="locator" xlink:label="us-gaap_DebtInstrumentConvertibleConversionPrice1" />
    <link:loc xlink:href="psn-20220331.xsd#psn_ConvertibleNotesConvertedEachIntoSharesOfCommonStock" xlink:type="locator" xlink:label="psn_ConvertibleNotesConvertedEachIntoSharesOfCommonStock" />
    <link:loc xlink:href="psn-20220331.xsd#psn_ConvertibleNoteConvertedEachAmount" xlink:type="locator" xlink:label="psn_ConvertibleNoteConvertedEachAmount" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentMaturityDate" xlink:type="locator" xlink:label="us-gaap_DebtInstrumentMaturityDate" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentFrequencyOfPeriodicPayment" xlink:type="locator" xlink:label="us-gaap_DebtInstrumentFrequencyOfPeriodicPayment" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentPaymentTerms" xlink:type="locator" xlink:label="us-gaap_DebtInstrumentPaymentTerms" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentFeeAmount" xlink:type="locator" xlink:label="us-gaap_DebtInstrumentFeeAmount" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ProceedsFromDebtNetOfIssuanceCosts" xlink:type="locator" xlink:label="us-gaap_ProceedsFromDebtNetOfIssuanceCosts" />
    <link:loc xlink:href="psn-20220331.xsd#psn_DebtInstrumentExerciseOfOptionByInitialPurchasersAggregatePrincipalAmount" xlink:type="locator" xlink:label="psn_DebtInstrumentExerciseOfOptionByInitialPurchasersAggregatePrincipalAmount" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_NotesPayableFairValueDisclosure" xlink:type="locator" xlink:label="us-gaap_NotesPayableFairValueDisclosure" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RepaymentsOfDebt" xlink:type="locator" xlink:label="us-gaap_RepaymentsOfDebt" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_InterestPayableCurrent" xlink:type="locator" xlink:label="us-gaap_InterestPayableCurrent" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentPeriodicPayment" xlink:type="locator" xlink:label="us-gaap_DebtInstrumentPeriodicPayment" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentFaceAmount" xlink:type="locator" xlink:label="us-gaap_DebtInstrumentFaceAmount" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentCarryingAmount" xlink:type="locator" xlink:label="us-gaap_DebtInstrumentCarryingAmount" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_InterestExpenseDebt" xlink:type="locator" xlink:label="us-gaap_InterestExpenseDebt" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LettersOfCreditOutstandingAmount" xlink:type="locator" xlink:label="us-gaap_LettersOfCreditOutstandingAmount" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage" xlink:type="locator" xlink:label="us-gaap_DebtInstrumentInterestRateStatedPercentage" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentBasisSpreadOnVariableRate1" xlink:type="locator" xlink:label="us-gaap_DebtInstrumentBasisSpreadOnVariableRate1" />
    <link:loc xlink:href="psn-20220331.xsd#psn_LineOfCreditFacilityNumberOfExtensions" xlink:type="locator" xlink:label="psn_LineOfCreditFacilityNumberOfExtensions" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LineOfCreditFacilityExpirationPeriod" xlink:type="locator" xlink:label="us-gaap_LineOfCreditFacilityExpirationPeriod" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity" xlink:type="locator" xlink:label="us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DeferredFinanceCostsNet" xlink:type="locator" xlink:label="us-gaap_DeferredFinanceCostsNet" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity" xlink:type="locator" xlink:label="us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LineOfCreditFacilityInitiationDate1" xlink:type="locator" xlink:label="us-gaap_LineOfCreditFacilityInitiationDate1" />
    <link:loc xlink:href="psn-20220331.xsd#psn_ConvertibleNoteHedgeAndWarrantTransactionMember" xlink:type="locator" xlink:label="psn_ConvertibleNoteHedgeAndWarrantTransactionMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AccountingStandardsUpdate202006Member" xlink:type="locator" xlink:label="us-gaap_AccountingStandardsUpdate202006Member" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TypeOfAdoptionMember" xlink:type="locator" xlink:label="us-gaap_TypeOfAdoptionMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AdjustmentsForNewAccountingPronouncementsAxis" xlink:type="locator" xlink:label="us-gaap_AdjustmentsForNewAccountingPronouncementsAxis" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ConvertibleDebtMember" xlink:type="locator" xlink:label="us-gaap_ConvertibleDebtMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueInputsLevel2Member" xlink:type="locator" xlink:label="us-gaap_FairValueInputsLevel2Member" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:type="locator" xlink:label="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:type="locator" xlink:label="us-gaap_FairValueMeasurementsFairValueHierarchyDomain_2" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:type="locator" xlink:label="us-gaap_FairValueByFairValueHierarchyLevelAxis" />
    <link:loc xlink:href="psn-20220331.xsd#psn_AccruedExpensesAndOtherCurrentLiabilitiesMember" xlink:type="locator" xlink:label="psn_AccruedExpensesAndOtherCurrentLiabilitiesMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BalanceSheetLocationDomain" xlink:type="locator" xlink:label="us-gaap_BalanceSheetLocationDomain" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BalanceSheetLocationDomain" xlink:type="locator" xlink:label="us-gaap_BalanceSheetLocationDomain_2" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BalanceSheetLocationAxis" xlink:type="locator" xlink:label="us-gaap_BalanceSheetLocationAxis" />
    <link:loc xlink:href="psn-20220331.xsd#psn_SeriesASeniorNoteMember" xlink:type="locator" xlink:label="psn_SeriesASeniorNoteMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SeniorNotesMember" xlink:type="locator" xlink:label="us-gaap_SeniorNotesMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LongtermDebtTypeDomain" xlink:type="locator" xlink:label="us-gaap_LongtermDebtTypeDomain" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LongtermDebtTypeAxis" xlink:type="locator" xlink:label="us-gaap_LongtermDebtTypeAxis" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PrivatePlacementMember" xlink:type="locator" xlink:label="us-gaap_PrivatePlacementMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SaleOfStockNameOfTransactionDomain" xlink:type="locator" xlink:label="us-gaap_SaleOfStockNameOfTransactionDomain" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SaleOfStockNameOfTransactionDomain" xlink:type="locator" xlink:label="us-gaap_SaleOfStockNameOfTransactionDomain_2" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SubsidiarySaleOfStockAxis" xlink:type="locator" xlink:label="us-gaap_SubsidiarySaleOfStockAxis" />
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_MaximumMember" xlink:type="locator" xlink:label="srt_MaximumMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_MinimumMember" xlink:type="locator" xlink:label="srt_MinimumMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeMember" xlink:type="locator" xlink:label="srt_RangeMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeAxis" xlink:type="locator" xlink:label="srt_RangeAxis" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EurodollarMember" xlink:type="locator" xlink:label="us-gaap_EurodollarMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BaseRateMember" xlink:type="locator" xlink:label="us-gaap_BaseRateMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_VariableRateDomain" xlink:type="locator" xlink:label="us-gaap_VariableRateDomain" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_VariableRateDomain" xlink:type="locator" xlink:label="us-gaap_VariableRateDomain_2" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_VariableRateAxis" xlink:type="locator" xlink:label="us-gaap_VariableRateAxis" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevolvingCreditFacilityMember" xlink:type="locator" xlink:label="us-gaap_RevolvingCreditFacilityMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CreditFacilityDomain" xlink:type="locator" xlink:label="us-gaap_CreditFacilityDomain" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CreditFacilityDomain" xlink:type="locator" xlink:label="us-gaap_CreditFacilityDomain_2" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CreditFacilityAxis" xlink:type="locator" xlink:label="us-gaap_CreditFacilityAxis" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentTable" xlink:type="locator" xlink:label="us-gaap_DebtInstrumentTable" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentLineItems" xlink:type="locator" xlink:label="us-gaap_DebtInstrumentLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_DebtInstrumentTable" order="10010.00" priority="2" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_DebtInstrumentTable" xlink:to="us-gaap_CreditFacilityAxis" order="10040.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_CreditFacilityAxis" xlink:to="us-gaap_CreditFacilityDomain_2" order="10050.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_CreditFacilityAxis" xlink:to="us-gaap_CreditFacilityDomain" order="10070.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_CreditFacilityDomain" xlink:to="us-gaap_RevolvingCreditFacilityMember" order="10090.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_DebtInstrumentTable" xlink:to="us-gaap_VariableRateAxis" order="10100.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_VariableRateAxis" xlink:to="us-gaap_VariableRateDomain_2" order="10110.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_VariableRateAxis" xlink:to="us-gaap_VariableRateDomain" order="10130.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_VariableRateDomain" xlink:to="us-gaap_BaseRateMember" order="10150.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_VariableRateDomain" xlink:to="us-gaap_EurodollarMember" order="10190.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_DebtInstrumentTable" xlink:to="srt_RangeAxis" order="10220.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="srt_RangeAxis" xlink:to="srt_RangeMember" order="10250.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_RangeMember" xlink:to="srt_MinimumMember" order="10270.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_RangeMember" xlink:to="srt_MaximumMember" order="10290.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_DebtInstrumentTable" xlink:to="us-gaap_SubsidiarySaleOfStockAxis" order="10340.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_SubsidiarySaleOfStockAxis" xlink:to="us-gaap_SaleOfStockNameOfTransactionDomain_2" order="10350.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_SubsidiarySaleOfStockAxis" xlink:to="us-gaap_SaleOfStockNameOfTransactionDomain" order="10370.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SaleOfStockNameOfTransactionDomain" xlink:to="us-gaap_PrivatePlacementMember" order="10390.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_DebtInstrumentTable" xlink:to="us-gaap_LongtermDebtTypeAxis" order="10560.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_LongtermDebtTypeAxis" xlink:to="us-gaap_LongtermDebtTypeDomain" order="10590.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LongtermDebtTypeDomain" xlink:to="us-gaap_SeniorNotesMember" order="10610.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LongtermDebtTypeDomain" xlink:to="psn_SeriesASeniorNoteMember" order="10650.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_DebtInstrumentTable" xlink:to="us-gaap_BalanceSheetLocationAxis" order="10660.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_BalanceSheetLocationAxis" xlink:to="us-gaap_BalanceSheetLocationDomain_2" order="10670.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_BalanceSheetLocationAxis" xlink:to="us-gaap_BalanceSheetLocationDomain" order="10690.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BalanceSheetLocationDomain" xlink:to="psn_AccruedExpensesAndOtherCurrentLiabilitiesMember" order="10710.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_DebtInstrumentTable" xlink:to="us-gaap_FairValueByFairValueHierarchyLevelAxis" order="10720.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:to="us-gaap_FairValueMeasurementsFairValueHierarchyDomain_2" order="10730.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:to="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" order="10750.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:to="us-gaap_FairValueInputsLevel2Member" order="10770.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LongtermDebtTypeDomain" xlink:to="us-gaap_ConvertibleDebtMember" order="10830.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_DebtInstrumentTable" xlink:to="us-gaap_AdjustmentsForNewAccountingPronouncementsAxis" order="11500.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_AdjustmentsForNewAccountingPronouncementsAxis" xlink:to="us-gaap_TypeOfAdoptionMember" order="11530.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TypeOfAdoptionMember" xlink:to="us-gaap_AccountingStandardsUpdate202006Member" order="11550.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LongtermDebtTypeDomain" xlink:to="psn_ConvertibleNoteHedgeAndWarrantTransactionMember" order="11910.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_LineOfCreditFacilityInitiationDate1" order="12100.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity" order="12140.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_DeferredFinanceCostsNet" order="12180.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity" order="12220.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_LineOfCreditFacilityExpirationPeriod" order="12300.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="psn_LineOfCreditFacilityNumberOfExtensions" order="12340.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_DebtInstrumentBasisSpreadOnVariableRate1" order="12380.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_DebtInstrumentInterestRateStatedPercentage" order="12540.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_LettersOfCreditOutstandingAmount" order="12620.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_InterestExpenseDebt" order="12700.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_DebtInstrumentCarryingAmount" order="12780.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_DebtInstrumentFaceAmount" order="12850.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_DebtInstrumentPeriodicPayment" order="13080.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_InterestPayableCurrent" order="13200.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_RepaymentsOfDebt" order="13260.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_NotesPayableFairValueDisclosure" order="13350.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="psn_DebtInstrumentExerciseOfOptionByInitialPurchasersAggregatePrincipalAmount" order="13510.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_ProceedsFromDebtNetOfIssuanceCosts" order="13530.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_DebtInstrumentFeeAmount" order="13550.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_DebtInstrumentPaymentTerms" order="13590.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_DebtInstrumentFrequencyOfPeriodicPayment" order="13610.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_DebtInstrumentMaturityDate" order="13630.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="psn_ConvertibleNoteConvertedEachAmount" order="13680.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="psn_ConvertibleNotesConvertedEachIntoSharesOfCommonStock" order="13700.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_DebtInstrumentConvertibleConversionPrice1" order="13720.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_DebtInstrumentConvertibleThresholdTradingDays" order="13770.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_DebtInstrumentConvertibleThresholdConsecutiveTradingDays1" order="13790.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_DebtInstrumentConvertibleThresholdPercentageOfStockPriceTrigger" order="13810.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="psn_DebtInstrumentConvertibleBusinessPeriod" order="13860.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="psn_DebtInstrumentConvertibleConsecutiveBusinessDayPeriod" order="13880.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="psn_DebtInstrumentConvertiblePrincipalAmount" order="13900.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="psn_DebtInstrumentConvertibleMeasurementPeriodPercentage" order="13920.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="psn_PercentageOfPrincipalAmountToRedeemConvertibleSeniorNotes" order="13970.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="psn_PercentageOfConvertibleSeniorNotesRepurchasedAtCash" order="14060.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="psn_DebtInstrumentConvertibleDebtLiability" order="14110.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="psn_DebtInstrumentConvertibleNotesAdditionalPaidInCapital" order="14170.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_DebtInstrumentInterestRateEffectivePercentage" order="14290.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_ConvertibleDebt" order="14350.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="psn_ReclassificationOfEquityComponentOfConvertibleSeniorNotesToLiabilities" order="14410.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="psn_ReclassificationOfEquityComponentOfDebtIssuanceCostsToLiabilities" order="14470.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="psn_CarryingAmountOfConvertibleSeniorNotesAdjustedInRetainedEarnings" order="14530.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="psn_BondHedgeTerm" order="14620.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="psn_DebtInstrumentNumberOfSharesIssuableUponConversion" order="14640.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="psn_CostOfConvertibleNoteHedgeTransaction" order="14660.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="psn_NumberOfCommonStockAcquiredByOffsettingSaleWarrants" order="14710.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1" order="14730.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_ProceedsFromIssuanceOfWarrants" order="14750.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="psn_PercentageOfBondHedgeAndWarrantTransactionsIncreasedConversionPrice" order="14800.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="psn_NetCostOfPurchaseOfBondHedgesAndSaleOfWarrants" order="14910.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_DeferredIncomeTaxLiabilitiesNet" order="14960.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_DeferredIncomeTaxAssetsNet" order="14980.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="psn_ReversalOfDeferredTaxLiability" order="15000.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="psn_AdditionalDeferredTaxLiabilityRelatedToCapitalizedDebtIssuanceCost" order="15020.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="psn_AdjustmentToDeferredTaxAssetThroughRetainedEarnings" order="15040.00" priority="2" use="optional" />
  </link:definitionLink>
  <link:roleRef roleURI="http://www.parsons.com/20220331/taxonomy/role/DisclosureDebtAndCreditFacilitiesScheduleOfAggregateAmountOfDebtDetails" xlink:href="psn-20220331.xsd#Role_DisclosureDebtAndCreditFacilitiesScheduleOfAggregateAmountOfDebtDetails" xlink:type="simple" />
  <link:definitionLink xlink:type="extended" xlink:role="http://www.parsons.com/20220331/taxonomy/role/DisclosureDebtAndCreditFacilitiesScheduleOfAggregateAmountOfDebtDetails">
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentInterestRateDuringPeriod" xlink:type="locator" xlink:label="us-gaap_DebtInstrumentInterestRateDuringPeriod" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentMaturityDate" xlink:type="locator" xlink:label="us-gaap_DebtInstrumentMaturityDate" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentFaceAmount" xlink:type="locator" xlink:label="us-gaap_DebtInstrumentFaceAmount" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PrivatePlacementMember" xlink:type="locator" xlink:label="us-gaap_PrivatePlacementMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SaleOfStockNameOfTransactionDomain" xlink:type="locator" xlink:label="us-gaap_SaleOfStockNameOfTransactionDomain" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SubsidiarySaleOfStockAxis" xlink:type="locator" xlink:label="us-gaap_SubsidiarySaleOfStockAxis" />
    <link:loc xlink:href="psn-20220331.xsd#psn_SeriesDSeniorNoteMember" xlink:type="locator" xlink:label="psn_SeriesDSeniorNoteMember" />
    <link:loc xlink:href="psn-20220331.xsd#psn_SeriesCSeniorNoteMember" xlink:type="locator" xlink:label="psn_SeriesCSeniorNoteMember" />
    <link:loc xlink:href="psn-20220331.xsd#psn_SeriesBSeniorNoteMember" xlink:type="locator" xlink:label="psn_SeriesBSeniorNoteMember" />
    <link:loc xlink:href="psn-20220331.xsd#psn_SeriesASeniorNoteMember" xlink:type="locator" xlink:label="psn_SeriesASeniorNoteMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LongtermDebtTypeDomain" xlink:type="locator" xlink:label="us-gaap_LongtermDebtTypeDomain" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LongtermDebtTypeAxis" xlink:type="locator" xlink:label="us-gaap_LongtermDebtTypeAxis" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentTable" xlink:type="locator" xlink:label="us-gaap_DebtInstrumentTable" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentLineItems" xlink:type="locator" xlink:label="us-gaap_DebtInstrumentLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_DebtInstrumentTable" order="10010.00" priority="2" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_DebtInstrumentTable" xlink:to="us-gaap_LongtermDebtTypeAxis" order="10040.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_LongtermDebtTypeAxis" xlink:to="us-gaap_LongtermDebtTypeDomain" order="10070.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LongtermDebtTypeDomain" xlink:to="psn_SeriesASeniorNoteMember" order="10090.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LongtermDebtTypeDomain" xlink:to="psn_SeriesBSeniorNoteMember" order="10110.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LongtermDebtTypeDomain" xlink:to="psn_SeriesCSeniorNoteMember" order="10130.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LongtermDebtTypeDomain" xlink:to="psn_SeriesDSeniorNoteMember" order="10150.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_DebtInstrumentTable" xlink:to="us-gaap_SubsidiarySaleOfStockAxis" order="10160.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_SubsidiarySaleOfStockAxis" xlink:to="us-gaap_SaleOfStockNameOfTransactionDomain" order="10190.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SaleOfStockNameOfTransactionDomain" xlink:to="us-gaap_PrivatePlacementMember" order="10210.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_DebtInstrumentFaceAmount" order="10300.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_DebtInstrumentMaturityDate" order="10400.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentLineItems" xlink:to="us-gaap_DebtInstrumentInterestRateDuringPeriod" order="10500.00" priority="2" use="optional" />
  </link:definitionLink>
  <link:roleRef roleURI="http://www.parsons.com/20220331/taxonomy/role/DisclosureRetirementBenefitPlanAdditionalInformationDetails" xlink:href="psn-20220331.xsd#Role_DisclosureRetirementBenefitPlanAdditionalInformationDetails" xlink:type="simple" />
  <link:definitionLink xlink:type="extended" xlink:role="http://www.parsons.com/20220331/taxonomy/role/DisclosureRetirementBenefitPlanAdditionalInformationDetails">
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EmployeeStockOwnershipPlanESOPCompensationExpense" xlink:type="locator" xlink:label="us-gaap_EmployeeStockOwnershipPlanESOPCompensationExpense" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EmployeeStockOwnershipPlanESOPSharesInESOP" xlink:type="locator" xlink:label="us-gaap_EmployeeStockOwnershipPlanESOPSharesInESOP" />
    <link:loc xlink:href="psn-20220331.xsd#psn_CommonStockSharesOutstandingIncludingEmployeeStockOwnershipPlanShares" xlink:type="locator" xlink:label="psn_CommonStockSharesOutstandingIncludingEmployeeStockOwnershipPlanShares" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1" xlink:type="locator" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardDescription" xlink:type="locator" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardDescription" />
    <link:loc xlink:href="psn-20220331.xsd#psn_DirectCostsOfContractsAndSellingGeneralAndAdministrativeExpenseMember" xlink:type="locator" xlink:label="psn_DirectCostsOfContractsAndSellingGeneralAndAdministrativeExpenseMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:type="locator" xlink:label="us-gaap_IncomeStatementLocationDomain" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:type="locator" xlink:label="us-gaap_IncomeStatementLocationDomain_2" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeStatementLocationAxis" xlink:type="locator" xlink:label="us-gaap_IncomeStatementLocationAxis" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfEmployeeStockOwnershipPlanESOPDisclosuresTable" xlink:type="locator" xlink:label="us-gaap_ScheduleOfEmployeeStockOwnershipPlanESOPDisclosuresTable" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresLineItems" xlink:type="locator" xlink:label="us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresLineItems" xlink:to="us-gaap_ScheduleOfEmployeeStockOwnershipPlanESOPDisclosuresTable" order="10010.00" priority="2" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfEmployeeStockOwnershipPlanESOPDisclosuresTable" xlink:to="us-gaap_IncomeStatementLocationAxis" order="10040.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_IncomeStatementLocationAxis" xlink:to="us-gaap_IncomeStatementLocationDomain_2" order="10050.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_IncomeStatementLocationAxis" xlink:to="us-gaap_IncomeStatementLocationDomain" order="10070.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_IncomeStatementLocationDomain" xlink:to="psn_DirectCostsOfContractsAndSellingGeneralAndAdministrativeExpenseMember" order="10090.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardDescription" order="10120.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1" order="10160.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresLineItems" xlink:to="psn_CommonStockSharesOutstandingIncludingEmployeeStockOwnershipPlanShares" order="10200.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresLineItems" xlink:to="us-gaap_EmployeeStockOwnershipPlanESOPSharesInESOP" order="10240.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresLineItems" xlink:to="us-gaap_EmployeeStockOwnershipPlanESOPCompensationExpense" order="10310.00" priority="2" use="optional" />
  </link:definitionLink>
  <link:roleRef roleURI="http://www.parsons.com/20220331/taxonomy/role/DisclosureInvestmentsInAndAdvancesToJointVenturesAdditionalInformationDetails" xlink:href="psn-20220331.xsd#Role_DisclosureInvestmentsInAndAdvancesToJointVenturesAdditionalInformationDetails" xlink:type="simple" />
  <link:definitionLink xlink:type="extended" xlink:role="http://www.parsons.com/20220331/taxonomy/role/DisclosureInvestmentsInAndAdvancesToJointVenturesAdditionalInformationDetails">
    <link:loc xlink:href="psn-20220331.xsd#psn_DistributionsFromContributionsToEquityMethodInvestment" xlink:type="locator" xlink:label="psn_DistributionsFromContributionsToEquityMethodInvestment" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LettersOfCreditOutstandingAmount" xlink:type="locator" xlink:label="us-gaap_LettersOfCreditOutstandingAmount" />
    <link:loc xlink:href="psn-20220331.xsd#psn_UnconsolidatedJointVenturesMember" xlink:type="locator" xlink:label="psn_UnconsolidatedJointVenturesMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_EquityMethodInvesteeNameDomain" xlink:type="locator" xlink:label="srt_EquityMethodInvesteeNameDomain" />
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_EquityMethodInvesteeNameDomain" xlink:type="locator" xlink:label="srt_EquityMethodInvesteeNameDomain_2" />
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" xlink:type="locator" xlink:label="srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" />
    <link:loc xlink:href="psn-20220331.xsd#psn_ConsolidatedJointVenturesMember" xlink:type="locator" xlink:label="psn_ConsolidatedJointVenturesMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RelatedPartyDomain" xlink:type="locator" xlink:label="us-gaap_RelatedPartyDomain" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:type="locator" xlink:label="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfEquityMethodInvestmentsTable" xlink:type="locator" xlink:label="us-gaap_ScheduleOfEquityMethodInvestmentsTable" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfEquityMethodInvestmentsLineItems" xlink:type="locator" xlink:label="us-gaap_ScheduleOfEquityMethodInvestmentsLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="us-gaap_ScheduleOfEquityMethodInvestmentsTable" order="10010.00" priority="2" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfEquityMethodInvestmentsTable" xlink:to="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" order="10040.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:to="us-gaap_RelatedPartyDomain" order="10070.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RelatedPartyDomain" xlink:to="psn_ConsolidatedJointVenturesMember" order="10090.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfEquityMethodInvestmentsTable" xlink:to="srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" order="10100.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" xlink:to="srt_EquityMethodInvesteeNameDomain_2" order="10110.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" xlink:to="srt_EquityMethodInvesteeNameDomain" order="10130.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_EquityMethodInvesteeNameDomain" xlink:to="psn_UnconsolidatedJointVenturesMember" order="10150.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="us-gaap_LettersOfCreditOutstandingAmount" order="10180.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="psn_DistributionsFromContributionsToEquityMethodInvestment" order="10250.00" priority="2" use="optional" />
  </link:definitionLink>
  <link:roleRef roleURI="http://www.parsons.com/20220331/taxonomy/role/DisclosureInvestmentsInAndAdvancesToJointVenturesSummaryOfFinancialInformationForConsolidatedJointVenturesDetails" xlink:href="psn-20220331.xsd#Role_DisclosureInvestmentsInAndAdvancesToJointVenturesSummaryOfFinancialInformationForConsolidatedJointVenturesDetails" xlink:type="simple" />
  <link:definitionLink xlink:type="extended" xlink:role="http://www.parsons.com/20220331/taxonomy/role/DisclosureInvestmentsInAndAdvancesToJointVenturesSummaryOfFinancialInformationForConsolidatedJointVenturesDetails">
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_NetIncomeLossAttributableToNoncontrollingInterest" xlink:type="locator" xlink:label="us-gaap_NetIncomeLossAttributableToNoncontrollingInterest" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_NetIncomeLoss" xlink:type="locator" xlink:label="us-gaap_NetIncomeLoss" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CostOfGoodsAndServicesSold" xlink:type="locator" xlink:label="us-gaap_CostOfGoodsAndServicesSold" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax" xlink:type="locator" xlink:label="us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockholdersEquity" xlink:type="locator" xlink:label="us-gaap_StockholdersEquity" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_Liabilities" xlink:type="locator" xlink:label="us-gaap_Liabilities" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LiabilitiesCurrent" xlink:type="locator" xlink:label="us-gaap_LiabilitiesCurrent" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_Assets" xlink:type="locator" xlink:label="us-gaap_Assets" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AssetsNoncurrent" xlink:type="locator" xlink:label="us-gaap_AssetsNoncurrent" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AssetsCurrent" xlink:type="locator" xlink:label="us-gaap_AssetsCurrent" />
    <link:loc xlink:href="psn-20220331.xsd#psn_ConsolidatedJointVenturesMember" xlink:type="locator" xlink:label="psn_ConsolidatedJointVenturesMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RelatedPartyDomain" xlink:type="locator" xlink:label="us-gaap_RelatedPartyDomain" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:type="locator" xlink:label="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfEquityMethodInvestmentsTable" xlink:type="locator" xlink:label="us-gaap_ScheduleOfEquityMethodInvestmentsTable" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfEquityMethodInvestmentsLineItems" xlink:type="locator" xlink:label="us-gaap_ScheduleOfEquityMethodInvestmentsLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="us-gaap_ScheduleOfEquityMethodInvestmentsTable" order="10050.00" priority="2" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfEquityMethodInvestmentsTable" xlink:to="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" order="10080.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:to="us-gaap_RelatedPartyDomain" order="10110.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RelatedPartyDomain" xlink:to="psn_ConsolidatedJointVenturesMember" order="10130.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="us-gaap_AssetsCurrent" order="10380.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="us-gaap_AssetsNoncurrent" order="10420.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="us-gaap_Assets" order="10460.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="us-gaap_LiabilitiesCurrent" order="10500.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="us-gaap_Liabilities" order="10540.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="us-gaap_StockholdersEquity" order="10580.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax" order="10650.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="us-gaap_CostOfGoodsAndServicesSold" order="10690.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="us-gaap_NetIncomeLoss" order="10730.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="us-gaap_NetIncomeLossAttributableToNoncontrollingInterest" order="10770.00" priority="2" use="optional" />
  </link:definitionLink>
  <link:roleRef roleURI="http://www.parsons.com/20220331/taxonomy/role/DisclosureInvestmentsInAndAdvancesToJointVenturesSummaryOfFinancialInformationForUnconsolidatedJointVenturesDetails" xlink:href="psn-20220331.xsd#Role_DisclosureInvestmentsInAndAdvancesToJointVenturesSummaryOfFinancialInformationForUnconsolidatedJointVenturesDetails" xlink:type="simple" />
  <link:definitionLink xlink:type="extended" xlink:role="http://www.parsons.com/20220331/taxonomy/role/DisclosureInvestmentsInAndAdvancesToJointVenturesSummaryOfFinancialInformationForUnconsolidatedJointVenturesDetails">
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeLossFromEquityMethodInvestments" xlink:type="locator" xlink:label="us-gaap_IncomeLossFromEquityMethodInvestments" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_NetIncomeLoss" xlink:type="locator" xlink:label="us-gaap_NetIncomeLoss" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CostOfGoodsAndServicesSold" xlink:type="locator" xlink:label="us-gaap_CostOfGoodsAndServicesSold" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax" xlink:type="locator" xlink:label="us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures" xlink:type="locator" xlink:label="us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockholdersEquity" xlink:type="locator" xlink:label="us-gaap_StockholdersEquity" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_Liabilities" xlink:type="locator" xlink:label="us-gaap_Liabilities" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LiabilitiesNoncurrent" xlink:type="locator" xlink:label="us-gaap_LiabilitiesNoncurrent" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LiabilitiesCurrent" xlink:type="locator" xlink:label="us-gaap_LiabilitiesCurrent" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_Assets" xlink:type="locator" xlink:label="us-gaap_Assets" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AssetsNoncurrent" xlink:type="locator" xlink:label="us-gaap_AssetsNoncurrent" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AssetsCurrent" xlink:type="locator" xlink:label="us-gaap_AssetsCurrent" />
    <link:loc xlink:href="psn-20220331.xsd#psn_UnconsolidatedJointVenturesMember" xlink:type="locator" xlink:label="psn_UnconsolidatedJointVenturesMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_EquityMethodInvesteeNameDomain" xlink:type="locator" xlink:label="srt_EquityMethodInvesteeNameDomain" />
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" xlink:type="locator" xlink:label="srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfEquityMethodInvestmentsTable" xlink:type="locator" xlink:label="us-gaap_ScheduleOfEquityMethodInvestmentsTable" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfEquityMethodInvestmentsLineItems" xlink:type="locator" xlink:label="us-gaap_ScheduleOfEquityMethodInvestmentsLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="us-gaap_ScheduleOfEquityMethodInvestmentsTable" order="10030.00" priority="2" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfEquityMethodInvestmentsTable" xlink:to="srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" order="10060.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" xlink:to="srt_EquityMethodInvesteeNameDomain" order="10090.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_EquityMethodInvesteeNameDomain" xlink:to="psn_UnconsolidatedJointVenturesMember" order="10110.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="us-gaap_AssetsCurrent" order="10400.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="us-gaap_AssetsNoncurrent" order="10440.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="us-gaap_Assets" order="10480.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="us-gaap_LiabilitiesCurrent" order="10520.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="us-gaap_LiabilitiesNoncurrent" order="10560.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="us-gaap_Liabilities" order="10600.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="us-gaap_StockholdersEquity" order="10640.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVentures" order="10680.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax" order="10750.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="us-gaap_CostOfGoodsAndServicesSold" order="10790.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="us-gaap_NetIncomeLoss" order="10830.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ScheduleOfEquityMethodInvestmentsLineItems" xlink:to="us-gaap_IncomeLossFromEquityMethodInvestments" order="10870.00" priority="2" use="optional" />
  </link:definitionLink>
  <link:roleRef roleURI="http://www.parsons.com/20220331/taxonomy/role/DisclosureRelatedPartyTransactionsAdditionalInformationDetails" xlink:href="psn-20220331.xsd#Role_DisclosureRelatedPartyTransactionsAdditionalInformationDetails" xlink:type="simple" />
  <link:definitionLink xlink:type="extended" xlink:role="http://www.parsons.com/20220331/taxonomy/role/DisclosureRelatedPartyTransactionsAdditionalInformationDetails">
    <link:loc xlink:href="psn-20220331.xsd#psn_ReimbursableCostIncurred" xlink:type="locator" xlink:label="psn_ReimbursableCostIncurred" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RelatedPartyTransactionOtherRevenuesFromTransactionsWithRelatedParty" xlink:type="locator" xlink:label="us-gaap_RelatedPartyTransactionOtherRevenuesFromTransactionsWithRelatedParty" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_MajorityOwnedSubsidiaryUnconsolidatedMember" xlink:type="locator" xlink:label="us-gaap_MajorityOwnedSubsidiaryUnconsolidatedMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RelatedPartyDomain" xlink:type="locator" xlink:label="us-gaap_RelatedPartyDomain" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:type="locator" xlink:label="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:type="locator" xlink:label="us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RelatedPartyTransactionLineItems" xlink:type="locator" xlink:label="us-gaap_RelatedPartyTransactionLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_RelatedPartyTransactionLineItems" xlink:to="us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" order="10010.00" priority="2" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:to="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" order="10040.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:to="us-gaap_RelatedPartyDomain" order="10070.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RelatedPartyDomain" xlink:to="us-gaap_MajorityOwnedSubsidiaryUnconsolidatedMember" order="10090.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RelatedPartyTransactionLineItems" xlink:to="us-gaap_RelatedPartyTransactionOtherRevenuesFromTransactionsWithRelatedParty" order="10120.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RelatedPartyTransactionLineItems" xlink:to="psn_ReimbursableCostIncurred" order="10160.00" priority="2" use="optional" />
  </link:definitionLink>
  <link:roleRef roleURI="http://www.parsons.com/20220331/taxonomy/role/DisclosureRelatedPartyTransactionsScheduleOfConsolidatedBalanceSheetRelatedToServicesProvidedToUnconsolidatedJointVenturesDetails" xlink:href="psn-20220331.xsd#Role_DisclosureRelatedPartyTransactionsScheduleOfConsolidatedBalanceSheetRelatedToServicesProvidedToUnconsolidatedJointVenturesDetails" xlink:type="simple" />
  <link:definitionLink xlink:type="extended" xlink:role="http://www.parsons.com/20220331/taxonomy/role/DisclosureRelatedPartyTransactionsScheduleOfConsolidatedBalanceSheetRelatedToServicesProvidedToUnconsolidatedJointVenturesDetails">
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ContractWithCustomerLiabilityCurrent" xlink:type="locator" xlink:label="us-gaap_ContractWithCustomerLiabilityCurrent" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ContractWithCustomerAssetNetCurrent" xlink:type="locator" xlink:label="us-gaap_ContractWithCustomerAssetNetCurrent" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AccountsReceivableRelatedPartiesCurrent" xlink:type="locator" xlink:label="us-gaap_AccountsReceivableRelatedPartiesCurrent" />
    <link:loc xlink:href="psn-20220331.xsd#psn_UnconsolidatedJointVenturesMember" xlink:type="locator" xlink:label="psn_UnconsolidatedJointVenturesMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_EquityMethodInvesteeNameDomain" xlink:type="locator" xlink:label="srt_EquityMethodInvesteeNameDomain" />
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" xlink:type="locator" xlink:label="srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:type="locator" xlink:label="us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RelatedPartyTransactionLineItems" xlink:type="locator" xlink:label="us-gaap_RelatedPartyTransactionLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_RelatedPartyTransactionLineItems" xlink:to="us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" order="10010.00" priority="2" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:to="srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" order="10040.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis" xlink:to="srt_EquityMethodInvesteeNameDomain" order="10070.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_EquityMethodInvesteeNameDomain" xlink:to="psn_UnconsolidatedJointVenturesMember" order="10090.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RelatedPartyTransactionLineItems" xlink:to="us-gaap_AccountsReceivableRelatedPartiesCurrent" order="10160.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RelatedPartyTransactionLineItems" xlink:to="us-gaap_ContractWithCustomerAssetNetCurrent" order="10200.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RelatedPartyTransactionLineItems" xlink:to="us-gaap_ContractWithCustomerLiabilityCurrent" order="10240.00" priority="2" use="optional" />
  </link:definitionLink>
  <link:roleRef roleURI="http://www.parsons.com/20220331/taxonomy/role/DisclosureEarningsPerShareAdditionalInformationDetails" xlink:href="psn-20220331.xsd#Role_DisclosureEarningsPerShareAdditionalInformationDetails" xlink:type="simple" />
  <link:definitionLink xlink:type="extended" xlink:role="http://www.parsons.com/20220331/taxonomy/role/DisclosureEarningsPerShareAdditionalInformationDetails">
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1" xlink:type="locator" xlink:label="us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1" />
    <link:loc xlink:href="psn-20220331.xsd#psn_StockRepurchaseProgramCommencementDate" xlink:type="locator" xlink:label="psn_StockRepurchaseProgramCommencementDate" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockRepurchaseProgramNumberOfSharesAuthorizedToBeRepurchased" xlink:type="locator" xlink:label="us-gaap_StockRepurchaseProgramNumberOfSharesAuthorizedToBeRepurchased" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount" xlink:type="locator" xlink:label="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockCompensationPlanMember" xlink:type="locator" xlink:label="us-gaap_StockCompensationPlanMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AntidilutiveSecuritiesNameDomain" xlink:type="locator" xlink:label="us-gaap_AntidilutiveSecuritiesNameDomain" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AntidilutiveSecuritiesNameDomain" xlink:type="locator" xlink:label="us-gaap_AntidilutiveSecuritiesNameDomain_2" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis" xlink:type="locator" xlink:label="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable" xlink:type="locator" xlink:label="us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems" xlink:type="locator" xlink:label="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems" xlink:to="us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable" order="10010.00" priority="2" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable" xlink:to="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis" order="10040.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis" xlink:to="us-gaap_AntidilutiveSecuritiesNameDomain_2" order="10050.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis" xlink:to="us-gaap_AntidilutiveSecuritiesNameDomain" order="10070.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AntidilutiveSecuritiesNameDomain" xlink:to="us-gaap_StockCompensationPlanMember" order="10090.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems" xlink:to="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount" order="10120.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems" xlink:to="us-gaap_StockRepurchaseProgramNumberOfSharesAuthorizedToBeRepurchased" order="10190.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems" xlink:to="psn_StockRepurchaseProgramCommencementDate" order="10230.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems" xlink:to="us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1" order="10300.00" priority="2" use="optional" />
  </link:definitionLink>
  <link:roleRef roleURI="http://www.parsons.com/20220331/taxonomy/role/DisclosureSegmentInformationAdditionalInformationDetails" xlink:href="psn-20220331.xsd#Role_DisclosureSegmentInformationAdditionalInformationDetails" xlink:type="simple" />
  <link:definitionLink xlink:type="extended" xlink:role="http://www.parsons.com/20220331/taxonomy/role/DisclosureSegmentInformationAdditionalInformationDetails">
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentNet" xlink:type="locator" xlink:label="us-gaap_PropertyPlantAndEquipmentNet" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax" xlink:type="locator" xlink:label="us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_NumberOfReportableSegments" xlink:type="locator" xlink:label="us-gaap_NumberOfReportableSegments" />
    <link:loc xlink:href="https://xbrl.sec.gov/country/2021/country-2021.xsd#country_US" xlink:type="locator" xlink:label="country_US" />
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_SegmentGeographicalDomain" xlink:type="locator" xlink:label="srt_SegmentGeographicalDomain" />
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_SegmentGeographicalDomain" xlink:type="locator" xlink:label="srt_SegmentGeographicalDomain_2" />
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_StatementGeographicalAxis" xlink:type="locator" xlink:label="srt_StatementGeographicalAxis" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable" xlink:type="locator" xlink:label="us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SegmentReportingInformationLineItems" xlink:type="locator" xlink:label="us-gaap_SegmentReportingInformationLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_SegmentReportingInformationLineItems" xlink:to="us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable" order="10010.00" priority="2" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable" xlink:to="srt_StatementGeographicalAxis" order="10040.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="srt_StatementGeographicalAxis" xlink:to="srt_SegmentGeographicalDomain_2" order="10050.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="srt_StatementGeographicalAxis" xlink:to="srt_SegmentGeographicalDomain" order="10070.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_SegmentGeographicalDomain" xlink:to="country_US" order="10090.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SegmentReportingInformationLineItems" xlink:to="us-gaap_NumberOfReportableSegments" order="10140.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SegmentReportingInformationLineItems" xlink:to="us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax" order="10190.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SegmentReportingInformationLineItems" xlink:to="us-gaap_PropertyPlantAndEquipmentNet" order="10230.00" priority="2" use="optional" />
  </link:definitionLink>
  <link:roleRef roleURI="http://www.parsons.com/20220331/taxonomy/role/DisclosureSegmentInformationSummaryOfBusinessSegmentInformationDetails" xlink:href="psn-20220331.xsd#Role_DisclosureSegmentInformationSummaryOfBusinessSegmentInformationDetails" xlink:type="simple" />
  <link:definitionLink xlink:type="extended" xlink:role="http://www.parsons.com/20220331/taxonomy/role/DisclosureSegmentInformationSummaryOfBusinessSegmentInformationDetails">
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax" xlink:type="locator" xlink:label="us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax" />
    <link:loc xlink:href="psn-20220331.xsd#psn_CriticalInfrastructureSegmentMember" xlink:type="locator" xlink:label="psn_CriticalInfrastructureSegmentMember" />
    <link:loc xlink:href="psn-20220331.xsd#psn_FederalSolutionSegmentMember" xlink:type="locator" xlink:label="psn_FederalSolutionSegmentMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SegmentDomain" xlink:type="locator" xlink:label="us-gaap_SegmentDomain" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementBusinessSegmentsAxis" xlink:type="locator" xlink:label="us-gaap_StatementBusinessSegmentsAxis" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable" xlink:type="locator" xlink:label="us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SegmentReportingInformationLineItems" xlink:type="locator" xlink:label="us-gaap_SegmentReportingInformationLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_SegmentReportingInformationLineItems" xlink:to="us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable" order="10010.00" priority="2" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable" xlink:to="us-gaap_StatementBusinessSegmentsAxis" order="10040.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementBusinessSegmentsAxis" xlink:to="us-gaap_SegmentDomain" order="10070.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SegmentDomain" xlink:to="psn_FederalSolutionSegmentMember" order="10090.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SegmentDomain" xlink:to="psn_CriticalInfrastructureSegmentMember" order="10110.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SegmentReportingInformationLineItems" xlink:to="us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax" order="10140.00" priority="2" use="optional" />
  </link:definitionLink>
  <link:roleRef roleURI="http://www.parsons.com/20220331/taxonomy/role/DisclosureSegmentInformationSummaryOfAdjustedEBITDABusinessSegmentInformationDetails" xlink:href="psn-20220331.xsd#Role_DisclosureSegmentInformationSummaryOfAdjustedEBITDABusinessSegmentInformationDetails" xlink:type="simple" />
  <link:definitionLink xlink:type="extended" xlink:role="http://www.parsons.com/20220331/taxonomy/role/DisclosureSegmentInformationSummaryOfAdjustedEBITDABusinessSegmentInformationDetails">
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_NetIncomeLoss" xlink:type="locator" xlink:label="us-gaap_NetIncomeLoss" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_NetIncomeLossAttributableToNoncontrollingInterest" xlink:type="locator" xlink:label="us-gaap_NetIncomeLossAttributableToNoncontrollingInterest" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ProfitLoss" xlink:type="locator" xlink:label="us-gaap_ProfitLoss" />
    <link:loc xlink:href="psn-20220331.xsd#psn_OtherReconciliationFromNetIncomeToAdjustedEBITDA" xlink:type="locator" xlink:label="psn_OtherReconciliationFromNetIncomeToAdjustedEBITDA" />
    <link:loc xlink:href="psn-20220331.xsd#psn_RestructuringIncomeExpense" xlink:type="locator" xlink:label="psn_RestructuringIncomeExpense" />
    <link:loc xlink:href="psn-20220331.xsd#psn_TransactionRelatedCosts" xlink:type="locator" xlink:label="psn_TransactionRelatedCosts" />
    <link:loc xlink:href="psn-20220331.xsd#psn_EquityBasedCompensationIncomeExpense" xlink:type="locator" xlink:label="psn_EquityBasedCompensationIncomeExpense" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeTaxExpenseBenefit" xlink:type="locator" xlink:label="us-gaap_IncomeTaxExpenseBenefit" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_InterestIncomeExpenseNet" xlink:type="locator" xlink:label="us-gaap_InterestIncomeExpenseNet" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DepreciationAndAmortization" xlink:type="locator" xlink:label="us-gaap_DepreciationAndAmortization" />
    <link:loc xlink:href="psn-20220331.xsd#psn_AdjustedEBITDAAttributableToNoncontrollingInterests" xlink:type="locator" xlink:label="psn_AdjustedEBITDAAttributableToNoncontrollingInterests" />
    <link:loc xlink:href="psn-20220331.xsd#psn_AdjustedEBITDAAttributableToParent" xlink:type="locator" xlink:label="psn_AdjustedEBITDAAttributableToParent" />
    <link:loc xlink:href="psn-20220331.xsd#psn_ReconciliationFromNetIncomeToAdjustedEBITDAAbstract" xlink:type="locator" xlink:label="psn_ReconciliationFromNetIncomeToAdjustedEBITDAAbstract" />
    <link:loc xlink:href="psn-20220331.xsd#psn_CriticalInfrastructureSegmentMember" xlink:type="locator" xlink:label="psn_CriticalInfrastructureSegmentMember" />
    <link:loc xlink:href="psn-20220331.xsd#psn_FederalSolutionSegmentMember" xlink:type="locator" xlink:label="psn_FederalSolutionSegmentMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SegmentDomain" xlink:type="locator" xlink:label="us-gaap_SegmentDomain" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementBusinessSegmentsAxis" xlink:type="locator" xlink:label="us-gaap_StatementBusinessSegmentsAxis" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable" xlink:type="locator" xlink:label="us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SegmentReportingInformationLineItems" xlink:type="locator" xlink:label="us-gaap_SegmentReportingInformationLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_SegmentReportingInformationLineItems" xlink:to="us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable" order="10100.00" priority="2" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable" xlink:to="us-gaap_StatementBusinessSegmentsAxis" order="10130.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementBusinessSegmentsAxis" xlink:to="us-gaap_SegmentDomain" order="10160.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SegmentDomain" xlink:to="psn_FederalSolutionSegmentMember" order="10180.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SegmentDomain" xlink:to="psn_CriticalInfrastructureSegmentMember" order="10200.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SegmentReportingInformationLineItems" xlink:to="psn_ReconciliationFromNetIncomeToAdjustedEBITDAAbstract" order="10230.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="psn_ReconciliationFromNetIncomeToAdjustedEBITDAAbstract" xlink:to="psn_AdjustedEBITDAAttributableToParent" order="10310.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="psn_ReconciliationFromNetIncomeToAdjustedEBITDAAbstract" xlink:to="psn_AdjustedEBITDAAttributableToNoncontrollingInterests" order="10430.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="psn_ReconciliationFromNetIncomeToAdjustedEBITDAAbstract" xlink:to="us-gaap_DepreciationAndAmortization" order="10470.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="psn_ReconciliationFromNetIncomeToAdjustedEBITDAAbstract" xlink:to="us-gaap_InterestIncomeExpenseNet" order="10510.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="psn_ReconciliationFromNetIncomeToAdjustedEBITDAAbstract" xlink:to="us-gaap_IncomeTaxExpenseBenefit" order="10550.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="psn_ReconciliationFromNetIncomeToAdjustedEBITDAAbstract" xlink:to="psn_EquityBasedCompensationIncomeExpense" order="10590.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="psn_ReconciliationFromNetIncomeToAdjustedEBITDAAbstract" xlink:to="psn_TransactionRelatedCosts" order="10630.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="psn_ReconciliationFromNetIncomeToAdjustedEBITDAAbstract" xlink:to="psn_RestructuringIncomeExpense" order="10670.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="psn_ReconciliationFromNetIncomeToAdjustedEBITDAAbstract" xlink:to="psn_OtherReconciliationFromNetIncomeToAdjustedEBITDA" order="10710.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="psn_ReconciliationFromNetIncomeToAdjustedEBITDAAbstract" xlink:to="us-gaap_ProfitLoss" order="10750.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="psn_ReconciliationFromNetIncomeToAdjustedEBITDAAbstract" xlink:to="us-gaap_NetIncomeLossAttributableToNoncontrollingInterest" order="10790.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="psn_ReconciliationFromNetIncomeToAdjustedEBITDAAbstract" xlink:to="us-gaap_NetIncomeLoss" order="10830.00" priority="2" use="optional" />
  </link:definitionLink>
  <link:roleRef roleURI="http://www.parsons.com/20220331/taxonomy/role/DisclosureSegmentInformationSummaryOfRevenuesAndPropertyAndEquipmentNetByGeographicAreaDetails" xlink:href="psn-20220331.xsd#Role_DisclosureSegmentInformationSummaryOfRevenuesAndPropertyAndEquipmentNetByGeographicAreaDetails" xlink:type="simple" />
  <link:definitionLink xlink:type="extended" xlink:role="http://www.parsons.com/20220331/taxonomy/role/DisclosureSegmentInformationSummaryOfRevenuesAndPropertyAndEquipmentNetByGeographicAreaDetails">
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentNet" xlink:type="locator" xlink:label="us-gaap_PropertyPlantAndEquipmentNet" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax" xlink:type="locator" xlink:label="us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_NonUsMember" xlink:type="locator" xlink:label="us-gaap_NonUsMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_MiddleEastMember" xlink:type="locator" xlink:label="us-gaap_MiddleEastMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_NorthAmericaMember" xlink:type="locator" xlink:label="srt_NorthAmericaMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_SegmentGeographicalDomain" xlink:type="locator" xlink:label="srt_SegmentGeographicalDomain" />
    <link:loc xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_StatementGeographicalAxis" xlink:type="locator" xlink:label="srt_StatementGeographicalAxis" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable" xlink:type="locator" xlink:label="us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SegmentReportingInformationLineItems" xlink:type="locator" xlink:label="us-gaap_SegmentReportingInformationLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_SegmentReportingInformationLineItems" xlink:to="us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable" order="10010.00" priority="2" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable" xlink:to="srt_StatementGeographicalAxis" order="10040.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="srt_StatementGeographicalAxis" xlink:to="srt_SegmentGeographicalDomain" order="10070.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_SegmentGeographicalDomain" xlink:to="srt_NorthAmericaMember" order="10090.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_SegmentGeographicalDomain" xlink:to="us-gaap_MiddleEastMember" order="10110.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="srt_SegmentGeographicalDomain" xlink:to="us-gaap_NonUsMember" order="10130.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SegmentReportingInformationLineItems" xlink:to="us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax" order="10240.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SegmentReportingInformationLineItems" xlink:to="us-gaap_PropertyPlantAndEquipmentNet" order="10430.00" priority="2" use="optional" />
  </link:definitionLink>
  <link:roleRef roleURI="http://www.parsons.com/20220331/taxonomy/role/DisclosureSegmentInformationSummaryOfRevenuesByBusinessLinesDetails" xlink:href="psn-20220331.xsd#Role_DisclosureSegmentInformationSummaryOfRevenuesByBusinessLinesDetails" xlink:type="simple" />
  <link:definitionLink xlink:type="extended" xlink:role="http://www.parsons.com/20220331/taxonomy/role/DisclosureSegmentInformationSummaryOfRevenuesByBusinessLinesDetails">
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax" xlink:type="locator" xlink:label="us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax" />
    <link:loc xlink:href="psn-20220331.xsd#psn_MobilitySolutionsMember" xlink:type="locator" xlink:label="psn_MobilitySolutionsMember" />
    <link:loc xlink:href="psn-20220331.xsd#psn_ConnectedCommunitiesMember" xlink:type="locator" xlink:label="psn_ConnectedCommunitiesMember" />
    <link:loc xlink:href="psn-20220331.xsd#psn_EngineeredSystemsMember" xlink:type="locator" xlink:label="psn_EngineeredSystemsMember" />
    <link:loc xlink:href="psn-20220331.xsd#psn_DefenseAndIntelligenceMember" xlink:type="locator" xlink:label="psn_DefenseAndIntelligenceMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SubsegmentsDomain" xlink:type="locator" xlink:label="us-gaap_SubsegmentsDomain" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SubsegmentsDomain" xlink:type="locator" xlink:label="us-gaap_SubsegmentsDomain_2" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SubsegmentsAxis" xlink:type="locator" xlink:label="us-gaap_SubsegmentsAxis" />
    <link:loc xlink:href="psn-20220331.xsd#psn_CriticalInfrastructureSegmentMember" xlink:type="locator" xlink:label="psn_CriticalInfrastructureSegmentMember" />
    <link:loc xlink:href="psn-20220331.xsd#psn_FederalSolutionSegmentMember" xlink:type="locator" xlink:label="psn_FederalSolutionSegmentMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SegmentDomain" xlink:type="locator" xlink:label="us-gaap_SegmentDomain" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementBusinessSegmentsAxis" xlink:type="locator" xlink:label="us-gaap_StatementBusinessSegmentsAxis" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable" xlink:type="locator" xlink:label="us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SegmentReportingInformationLineItems" xlink:type="locator" xlink:label="us-gaap_SegmentReportingInformationLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_SegmentReportingInformationLineItems" xlink:to="us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable" order="10010.00" priority="2" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable" xlink:to="us-gaap_StatementBusinessSegmentsAxis" order="10040.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementBusinessSegmentsAxis" xlink:to="us-gaap_SegmentDomain" order="10070.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SegmentDomain" xlink:to="psn_FederalSolutionSegmentMember" order="10090.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SegmentDomain" xlink:to="psn_CriticalInfrastructureSegmentMember" order="10150.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable" xlink:to="us-gaap_SubsegmentsAxis" order="10200.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_SubsegmentsAxis" xlink:to="us-gaap_SubsegmentsDomain_2" order="10210.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_SubsegmentsAxis" xlink:to="us-gaap_SubsegmentsDomain" order="10230.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SubsegmentsDomain" xlink:to="psn_DefenseAndIntelligenceMember" order="10250.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SubsegmentsDomain" xlink:to="psn_EngineeredSystemsMember" order="10270.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SubsegmentsDomain" xlink:to="psn_ConnectedCommunitiesMember" order="10290.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SubsegmentsDomain" xlink:to="psn_MobilitySolutionsMember" order="10310.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SegmentReportingInformationLineItems" xlink:to="us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax" order="10340.00" priority="2" use="optional" />
  </link:definitionLink>
  <link:roleRef roleURI="http://www.parsons.com/20220331/taxonomy/role/DisclosureSubsequentEventsAdditionalInformationDetails" xlink:href="psn-20220331.xsd#Role_DisclosureSubsequentEventsAdditionalInformationDetails" xlink:type="simple" />
  <link:definitionLink xlink:type="extended" xlink:role="http://www.parsons.com/20220331/taxonomy/role/DisclosureSubsequentEventsAdditionalInformationDetails">
    <link:loc xlink:href="psn-20220331.xsd#psn_NumberOfAnnualInstallments" xlink:type="locator" xlink:label="psn_NumberOfAnnualInstallments" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SubsequentEventMember" xlink:type="locator" xlink:label="us-gaap_SubsequentEventMember" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SubsequentEventTypeDomain" xlink:type="locator" xlink:label="us-gaap_SubsequentEventTypeDomain" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SubsequentEventTypeDomain" xlink:type="locator" xlink:label="us-gaap_SubsequentEventTypeDomain_2" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SubsequentEventTypeAxis" xlink:type="locator" xlink:label="us-gaap_SubsequentEventTypeAxis" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SubsequentEventTable" xlink:type="locator" xlink:label="us-gaap_SubsequentEventTable" />
    <link:loc xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SubsequentEventLineItems" xlink:type="locator" xlink:label="us-gaap_SubsequentEventLineItems" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_SubsequentEventLineItems" xlink:to="us-gaap_SubsequentEventTable" order="10010.00" priority="2" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_SubsequentEventTable" xlink:to="us-gaap_SubsequentEventTypeAxis" order="10040.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_SubsequentEventTypeAxis" xlink:to="us-gaap_SubsequentEventTypeDomain_2" order="10050.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_SubsequentEventTypeAxis" xlink:to="us-gaap_SubsequentEventTypeDomain" order="10070.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SubsequentEventTypeDomain" xlink:to="us-gaap_SubsequentEventMember" order="10090.00" priority="2" use="optional" />
    <link:definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SubsequentEventLineItems" xlink:to="psn_NumberOfAnnualInstallments" order="10120.00" priority="2" use="optional" />
  </link:definitionLink>
</link:linkbase>
