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<!-- DFIN New ActiveDisclosure (SM) XBRL Schema Document - http://www.dfinsolutions.com/ -->
<!-- Creation Date :2022-05-02T17:01:29.1215+00:00 -->
<!-- Copyright (c) 2022 Donnelly Financial Solutions, Inc. All Rights Reserved. -->
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  <xsd:element id="mksi_OptionAgreementMember" name="OptionAgreementMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="mksi_OriginalIssueDiscountMember" name="OriginalIssueDiscountMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="mksi_OtherAsiaMember" name="OtherAsiaMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="mksi_PatentsTrademarksTradeNamesAndOtherMember" name="PatentsTrademarksTradeNamesAndOtherMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="mksi_PhotonControlIncMember" name="PhotonControlIncMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="mksi_PhotonControlMember" name="PhotonControlMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="mksi_PhotonicsSolutionsDivisionMember" name="PhotonicsSolutionsDivisionMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="mksi_SeniorSecuredRevolvingCreditFacilityMember" name="SeniorSecuredRevolvingCreditFacilityMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="mksi_SeniorSecuredTermLoanFacilityMember" name="SeniorSecuredTermLoanFacilityMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="mksi_StockIncentivePlanTwoThousandFourteenMember" name="StockIncentivePlanTwoThousandFourteenMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="mksi_SwapAgreementOneMember" name="SwapAgreementOneMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="mksi_SwapAgreementThreeMember" name="SwapAgreementThreeMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="mksi_SwapAgreementTwoMember" name="SwapAgreementTwoMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="mksi_TermLoanCreditAgreementMember" name="TermLoanCreditAgreementMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="mksi_TermLoanFacilityMember" name="TermLoanFacilityMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="mksi_TermLoanFacilityNetMember" name="TermLoanFacilityNetMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="mksi_TermLoanMember" name="TermLoanMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="mksi_TimeDepositsAndCertificatesOfDepositMember" name="TimeDepositsAndCertificatesOfDepositMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="mksi_TrademarksTradenamesAndBacklogMember" name="TrademarksTradenamesAndBacklogMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="mksi_TrancheMember" name="TrancheMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="mksi_TwentyTwentyTwoNewRevolvingCreditFacilityMember" name="TwentyTwentyTwoNewRevolvingCreditFacilityMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="mksi_TwentyTwentyTwoNewTermLoanAFacilityMember" name="TwentyTwentyTwoNewTermLoanAFacilityMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="mksi_TwentyTwentyTwoNewTermLoanBFacilityMember" name="TwentyTwentyTwoNewTermLoanBFacilityMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="mksi_USDollarEuroMember" name="USDollarEuroMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="mksi_USDollarJapaneseYenMember" name="USDollarJapaneseYenMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="mksi_USDollarKoreanWonMember" name="USDollarKoreanWonMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="mksi_USDollarTaiwaneseDollarsMember" name="USDollarTaiwaneseDollarsMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="mksi_UsDollarUkPoundMember" name="UsDollarUkPoundMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="mksi_VacuumAndAnalysisSegmentMember" name="VacuumAndAnalysisSegmentMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="mksi_VacuumSolutionsDivisionMember" name="VacuumSolutionsDivisionMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="mksi_ChangeInContractWithCustomerLiabilityTable" name="ChangeInContractWithCustomerLiabilityTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="mksi_DerivativeAndHedgingTable" name="DerivativeAndHedgingTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="mksi_GoodwillAndIntangibleAssetsTable" name="GoodwillAndIntangibleAssetsTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="mksi_LeasesTable" name="LeasesTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="mksi_ScheduleOfEarningsPerShareTable" name="ScheduleOfEarningsPerShareTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="mksi_ScheduleOfRestructuringAndOtherCostTable" name="ScheduleOfRestructuringAndOtherCostTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="mksi_StockholdersEquityTable" name="StockholdersEquityTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="mksi_ExpenseOfRestructuringActivitiesAndOtherTextBlock" name="ExpenseOfRestructuringActivitiesAndOtherTextBlock" type="dtr-types:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false"/>
  <xsd:element id="mksi_ScheduleOfAllocationOfPurchasePriceTableTextBlock" name="ScheduleOfAllocationOfPurchasePriceTableTextBlock" type="dtr-types:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false"/>
  <xsd:element id="mksi_ScheduleOfCapitalExpenditureDepreciationAndAmortizationAndAssetsReportableSegmentsTableTextBlock" name="ScheduleOfCapitalExpenditureDepreciationAndAmortizationAndAssetsReportableSegmentsTableTextBlock" type="dtr-types:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false"/>
</xsd:schema>
