<?xml version="1.0" encoding="UTF-8"?>

<!--XBRL Document Created with Wdesk from Workiva-->
<!--Copyright 2021 Workiva-->
<!--r:dfb53080-eebe-4eeb-bcf1-83e4dc9467cc,g:db96b202-de42-4178-aec5-db5ab4e028f4-->
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:prax="http://praxismedicines.com/20210331" xmlns:xbrli="http://www.xbrl.org/2003/instance" xmlns:nonnum="http://www.xbrl.org/dtr/type/non-numeric" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xmlns:num="http://www.xbrl.org/dtr/type/numeric" attributeFormDefault="unqualified" elementFormDefault="qualified" targetNamespace="http://praxismedicines.com/20210331">
  <xs:import namespace="http://fasb.org/us-gaap/2020-01-31" schemaLocation="http://xbrl.fasb.org/us-gaap/2020/elts/us-gaap-2020-01-31.xsd"/>
  <xs:import namespace="http://www.w3.org/1999/xlink" schemaLocation="http://www.xbrl.org/2003/xlink-2003-12-31.xsd"/>
  <xs:import namespace="http://www.xbrl.org/2003/instance" schemaLocation="http://www.xbrl.org/2003/xbrl-instance-2003-12-31.xsd"/>
  <xs:import namespace="http://www.xbrl.org/2003/linkbase" schemaLocation="http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd"/>
  <xs:import namespace="http://www.xbrl.org/dtr/type/non-numeric" schemaLocation="http://www.xbrl.org/dtr/type/nonNumeric-2009-12-16.xsd"/>
  <xs:import namespace="http://www.xbrl.org/dtr/type/numeric" schemaLocation="http://www.xbrl.org/dtr/type/numeric-2009-12-16.xsd"/>
  <xs:import namespace="http://xbrl.org/2005/xbrldt" schemaLocation="http://www.xbrl.org/2005/xbrldt-2005.xsd"/>
  <xs:import namespace="http://xbrl.sec.gov/dei/2020-01-31" schemaLocation="https://xbrl.sec.gov/dei/2020/dei-2020-01-31.xsd"/>
  <xs:annotation>
    <xs:appinfo>
      <link:linkbaseRef xmlns:xlink="http://www.w3.org/1999/xlink" xlink:arcrole="http://www.w3.org/1999/xlink/properties/linkbase" xlink:href="prax-20210331_lab.xml" xlink:role="http://www.xbrl.org/2003/role/labelLinkbaseRef" xlink:type="simple"/>
      <link:linkbaseRef xmlns:xlink="http://www.w3.org/1999/xlink" xlink:arcrole="http://www.w3.org/1999/xlink/properties/linkbase" xlink:href="prax-20210331_cal.xml" xlink:role="http://www.xbrl.org/2003/role/calculationLinkbaseRef" xlink:type="simple"/>
      <link:linkbaseRef xmlns:xlink="http://www.w3.org/1999/xlink" xlink:arcrole="http://www.w3.org/1999/xlink/properties/linkbase" xlink:href="prax-20210331_def.xml" xlink:role="http://www.xbrl.org/2003/role/definitionLinkbaseRef" xlink:type="simple"/>
      <link:linkbaseRef xmlns:xlink="http://www.w3.org/1999/xlink" xlink:arcrole="http://www.w3.org/1999/xlink/properties/linkbase" xlink:href="prax-20210331_pre.xml" xlink:role="http://www.xbrl.org/2003/role/presentationLinkbaseRef" xlink:type="simple"/>
      <link:roleType id="CoverPage" roleURI="http://praxismedicines.com/role/CoverPage">
        <link:definition>0001001 - Document - Cover Page</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CondensedConsolidatedBalanceSheets" roleURI="http://praxismedicines.com/role/CondensedConsolidatedBalanceSheets">
        <link:definition>1001002 - Statement - Condensed Consolidated Balance Sheets</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CondensedConsolidatedBalanceSheetsParenthetical" roleURI="http://praxismedicines.com/role/CondensedConsolidatedBalanceSheetsParenthetical">
        <link:definition>1002003 - Statement - Condensed Consolidated Balance Sheets (Parenthetical)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CondensedConsolidatedStatementsofOperations" roleURI="http://praxismedicines.com/role/CondensedConsolidatedStatementsofOperations">
        <link:definition>1003004 - Statement - Condensed Consolidated Statements of Operations</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CondensedConsolidatedStatementsofComprehensiveLoss" roleURI="http://praxismedicines.com/role/CondensedConsolidatedStatementsofComprehensiveLoss">
        <link:definition>1004005 - Statement - Condensed Consolidated Statements of Comprehensive Loss</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="ConsolidatedStatementsofRedeemableConvertiblePreferredStockandStockholdersDeficitEquity" roleURI="http://praxismedicines.com/role/ConsolidatedStatementsofRedeemableConvertiblePreferredStockandStockholdersDeficitEquity">
        <link:definition>1005006 - Statement - Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' (Deficit) Equity</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CondensedConsolidatedStatementsofCashFlows" roleURI="http://praxismedicines.com/role/CondensedConsolidatedStatementsofCashFlows">
        <link:definition>1006007 - Statement - Condensed Consolidated Statements of Cash Flows</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="NatureoftheBusiness" roleURI="http://praxismedicines.com/role/NatureoftheBusiness">
        <link:definition>2101101 - Disclosure - Nature of the Business</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="NatureoftheBusinessAdditionalInformationDetails" roleURI="http://praxismedicines.com/role/NatureoftheBusinessAdditionalInformationDetails">
        <link:definition>2402401 - Disclosure - Nature of the Business - Additional Information (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="SummaryofSignificantAccountingPolicies" roleURI="http://praxismedicines.com/role/SummaryofSignificantAccountingPolicies">
        <link:definition>2103102 - Disclosure - Summary of Significant Accounting Policies</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="SummaryofSignificantAccountingPoliciesPolicies" roleURI="http://praxismedicines.com/role/SummaryofSignificantAccountingPoliciesPolicies">
        <link:definition>2204201 - Disclosure - Summary of Significant Accounting Policies (Policies)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="MarketableSecurities" roleURI="http://praxismedicines.com/role/MarketableSecurities">
        <link:definition>2105103 - Disclosure - Marketable Securities</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="MarketableSecuritiesTables" roleURI="http://praxismedicines.com/role/MarketableSecuritiesTables">
        <link:definition>2306301 - Disclosure - Marketable Securities (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="MarketableSecuritiesDetails" roleURI="http://praxismedicines.com/role/MarketableSecuritiesDetails">
        <link:definition>2407402 - Disclosure - Marketable Securities (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="FairValueMeasurements" roleURI="http://praxismedicines.com/role/FairValueMeasurements">
        <link:definition>2108104 - Disclosure - Fair Value Measurements</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="FairValueMeasurementsTables" roleURI="http://praxismedicines.com/role/FairValueMeasurementsTables">
        <link:definition>2309302 - Disclosure - Fair Value Measurements (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="FairValueMeasurementsSummaryofFinancialAssetsMeasuredatFairValueonaRecurringBasisDetails" roleURI="http://praxismedicines.com/role/FairValueMeasurementsSummaryofFinancialAssetsMeasuredatFairValueonaRecurringBasisDetails">
        <link:definition>2410403 - Disclosure - Fair Value Measurements - Summary of Financial Assets Measured at Fair Value on a Recurring Basis (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="AccruedExpenses" roleURI="http://praxismedicines.com/role/AccruedExpenses">
        <link:definition>2111105 - Disclosure - Accrued Expenses</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="AccruedExpensesTables" roleURI="http://praxismedicines.com/role/AccruedExpensesTables">
        <link:definition>2312303 - Disclosure - Accrued Expenses (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="AccruedExpensesSummaryofAccruedExpensesDetails" roleURI="http://praxismedicines.com/role/AccruedExpensesSummaryofAccruedExpensesDetails">
        <link:definition>2413404 - Disclosure - Accrued Expenses - Summary of Accrued Expenses (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CommitmentsandContingencies" roleURI="http://praxismedicines.com/role/CommitmentsandContingencies">
        <link:definition>2114106 - Disclosure - Commitments and Contingencies</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CommitmentsandContingenciesAdditionalInformationDetails" roleURI="http://praxismedicines.com/role/CommitmentsandContingenciesAdditionalInformationDetails">
        <link:definition>2415405 - Disclosure - Commitments and Contingencies - Additional Information (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CommonStockandPreferredStock" roleURI="http://praxismedicines.com/role/CommonStockandPreferredStock">
        <link:definition>2116107 - Disclosure - Common Stock and Preferred Stock</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CommonStockandPreferredStockTables" roleURI="http://praxismedicines.com/role/CommonStockandPreferredStockTables">
        <link:definition>2317304 - Disclosure - Common Stock and Preferred Stock (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CommonStockandPreferredStockSummaryofCommonStockReservedForFutureIssuanceDetails" roleURI="http://praxismedicines.com/role/CommonStockandPreferredStockSummaryofCommonStockReservedForFutureIssuanceDetails">
        <link:definition>2418406 - Disclosure - Common Stock and Preferred Stock - Summary of Common Stock Reserved For Future Issuance (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CommonStockandPreferredStockAdditionalInformationDetails" roleURI="http://praxismedicines.com/role/CommonStockandPreferredStockAdditionalInformationDetails">
        <link:definition>2419407 - Disclosure - Common Stock and Preferred Stock - Additional Information (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="StockBasedCompensation" roleURI="http://praxismedicines.com/role/StockBasedCompensation">
        <link:definition>2120108 - Disclosure - Stock-Based Compensation</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="StockBasedCompensationTables" roleURI="http://praxismedicines.com/role/StockBasedCompensationTables">
        <link:definition>2321305 - Disclosure - Stock-Based Compensation (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="StockBasedCompensationAdditionalInformationDetails" roleURI="http://praxismedicines.com/role/StockBasedCompensationAdditionalInformationDetails">
        <link:definition>2422408 - Disclosure - Stock-Based Compensation - Additional Information (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="StockBasedCompensationSummaryofRestrictedStockActivityDetails" roleURI="http://praxismedicines.com/role/StockBasedCompensationSummaryofRestrictedStockActivityDetails">
        <link:definition>2423409 - Disclosure - Stock-Based Compensation - Summary of Restricted Stock Activity (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="StockBasedCompensationSummaryofStockOptionActivityDetails" roleURI="http://praxismedicines.com/role/StockBasedCompensationSummaryofStockOptionActivityDetails">
        <link:definition>2424410 - Disclosure - Stock-Based Compensation - Summary of Stock Option Activity (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="StockBasedCompensationScheduleofFairValueofStockOptionAwardsontheGrantDateUsingtheBlackScholesOptionValuationModelDetails" roleURI="http://praxismedicines.com/role/StockBasedCompensationScheduleofFairValueofStockOptionAwardsontheGrantDateUsingtheBlackScholesOptionValuationModelDetails">
        <link:definition>2425411 - Disclosure - Stock-Based Compensation - Schedule of Fair Value of Stock Option Awards on the Grant Date Using the Black-Scholes Option Valuation Model (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="StockBasedCompensationSummaryofStockBasedCompensationExpenseDetails" roleURI="http://praxismedicines.com/role/StockBasedCompensationSummaryofStockBasedCompensationExpenseDetails">
        <link:definition>2426412 - Disclosure - Stock-Based Compensation - Summary of Stock-Based Compensation Expense (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="NetLossperShare" roleURI="http://praxismedicines.com/role/NetLossperShare">
        <link:definition>2127109 - Disclosure - Net Loss per Share</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="NetLossperShareTables" roleURI="http://praxismedicines.com/role/NetLossperShareTables">
        <link:definition>2328306 - Disclosure - Net Loss per Share (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="NetLossperShareSummaryofbasicanddilutednetlosspershareattributabletocommonstockholdersDetails" roleURI="http://praxismedicines.com/role/NetLossperShareSummaryofbasicanddilutednetlosspershareattributabletocommonstockholdersDetails">
        <link:definition>2429413 - Disclosure - Net Loss per Share - Summary of basic and diluted net loss per share attributable to common stockholders (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="NetLossperShareSummaryofantidilutivesecuritiesexcludedfromcomputationofearningspershareDetails" roleURI="http://praxismedicines.com/role/NetLossperShareSummaryofantidilutivesecuritiesexcludedfromcomputationofearningspershareDetails">
        <link:definition>2430414 - Disclosure - Net Loss per Share - Summary of antidilutive securities excluded from computation of earnings per share (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="RelatedPartyTransactions" roleURI="http://praxismedicines.com/role/RelatedPartyTransactions">
        <link:definition>2131110 - Disclosure - Related Party Transactions</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="RelatedPartyTransactionsAdditionalInformationDetails" roleURI="http://praxismedicines.com/role/RelatedPartyTransactionsAdditionalInformationDetails">
        <link:definition>2432415 - Disclosure - Related Party Transactions - Additional Information (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="SubsequentEvents" roleURI="http://praxismedicines.com/role/SubsequentEvents">
        <link:definition>2133111 - Disclosure - Subsequent Events</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
    </xs:appinfo>
  </xs:annotation>
  <xs:element id="prax_DebtSecuritiesAvailableForSaleAccumulatedGrossUnrealizedLossNoncurrentBeforeTax" abstract="false" name="DebtSecuritiesAvailableForSaleAccumulatedGrossUnrealizedLossNoncurrentBeforeTax" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="prax_TwentyTwentyEmployeeStockIncentivePlanMember" abstract="true" name="TwentyTwentyEmployeeStockIncentivePlanMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="nonnum:domainItemType"/>
  <xs:element id="prax_ScheduleOfCommonStockReservedForFutureIssuanceLineItems" abstract="true" name="ScheduleOfCommonStockReservedForFutureIssuanceLineItems" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:stringItemType"/>
  <xs:element id="prax_ExpenseReimbursementMember" abstract="true" name="ExpenseReimbursementMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="nonnum:domainItemType"/>
  <xs:element id="prax_TwentySeventeenEmployeeStockIncentivePlanMember" abstract="true" name="TwentySeventeenEmployeeStockIncentivePlanMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="nonnum:domainItemType"/>
  <xs:element id="prax_GainLossOnRepurchaseOfTemporaryEquity" abstract="false" name="GainLossOnRepurchaseOfTemporaryEquity" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="prax_AccruedPersonnelRelatedExpensesCurrent" abstract="false" name="AccruedPersonnelRelatedExpensesCurrent" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="prax_IncreaseDecreaseInOperatingLeaseLiabilities" abstract="false" name="IncreaseDecreaseInOperatingLeaseLiabilities" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="prax_OfficeSpaceInCambridgeMassachusettsMember" abstract="true" name="OfficeSpaceInCambridgeMassachusettsMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="nonnum:domainItemType"/>
  <xs:element id="prax_ScheduleOfCommonStockReservedForFutureIssuanceTable" abstract="true" name="ScheduleOfCommonStockReservedForFutureIssuanceTable" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrldt:hypercubeItem" type="xbrli:stringItemType"/>
  <xs:element id="prax_NumberOfProductCandidates" abstract="false" name="NumberOfProductCandidates" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="xbrli:integerItemType"/>
  <xs:element id="prax_SeriesCRedeemableConvertiblePreferredStockMember" abstract="true" name="SeriesCRedeemableConvertiblePreferredStockMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="nonnum:domainItemType"/>
  <xs:element id="prax_SeriesARedeemableConvertiblePreferredStockMember" abstract="true" name="SeriesARedeemableConvertiblePreferredStockMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="nonnum:domainItemType"/>
  <xs:element id="prax_SeriesBRedeemableConvertiblePreferredStockMember" abstract="true" name="SeriesBRedeemableConvertiblePreferredStockMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="nonnum:domainItemType"/>
  <xs:element id="prax_AdjustmentToRetainedEarningsRepurchaseOfConvertiblePreferredStock" abstract="false" name="AdjustmentToRetainedEarningsRepurchaseOfConvertiblePreferredStock" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="prax_DebtSecuritiesAvailableForSaleAccumulatedGrossUnrealizedLossCurrentBeforeTax" abstract="false" name="DebtSecuritiesAvailableForSaleAccumulatedGrossUnrealizedLossCurrentBeforeTax" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="prax_TemporaryEquityIssuanceCosts" abstract="false" name="TemporaryEquityIssuanceCosts" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="prax_ScheduleOfCommonStockReservedForFutureIssuanceTableTextBlock" abstract="false" name="ScheduleOfCommonStockReservedForFutureIssuanceTableTextBlock" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="nonnum:textBlockItemType"/>
  <xs:element id="prax_SeriesBOneRedeemableConvertiblePreferredStockMember" abstract="true" name="SeriesBOneRedeemableConvertiblePreferredStockMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="nonnum:domainItemType"/>
  <xs:element id="prax_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsIssuedInPeriod" abstract="false" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsIssuedInPeriod" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:sharesItemType"/>
  <xs:element id="prax_ConvertiblePreferredStocksMember" abstract="true" name="ConvertiblePreferredStocksMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="nonnum:domainItemType"/>
  <xs:element id="prax_DebtSecuritiesAvailableForSaleAccumulatedGrossUnrealizedGainCurrentBeforeTax" abstract="false" name="DebtSecuritiesAvailableForSaleAccumulatedGrossUnrealizedGainCurrentBeforeTax" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="prax_NetLossPerShareAbstract" abstract="true" name="NetLossPerShareAbstract" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:stringItemType"/>
  <xs:element id="prax_TwentyTwentyEmployeeStockPurchasePlanMember" abstract="true" name="TwentyTwentyEmployeeStockPurchasePlanMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="nonnum:domainItemType"/>
  <xs:element id="prax_TwoThousandTwentyEmployeeStockIncentivePlanMember" abstract="true" name="TwoThousandTwentyEmployeeStockIncentivePlanMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="nonnum:domainItemType"/>
  <xs:element id="prax_AccruedConsultingExpensesCurrent" abstract="false" name="AccruedConsultingExpensesCurrent" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="prax_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsIssuedInPeriodWeightedAverageGrantDateFairValue" abstract="false" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsIssuedInPeriodWeightedAverageGrantDateFairValue" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="num:perShareItemType"/>
  <xs:element id="prax_DebtSecuritiesAvailableForSaleAccumulatedGrossUnrealizedGainNoncurrentBeforeTax" abstract="false" name="DebtSecuritiesAvailableForSaleAccumulatedGrossUnrealizedGainNoncurrentBeforeTax" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="prax_ScheduleOfCommonStockReservedForFutureIssuanceAbstract" abstract="true" name="ScheduleOfCommonStockReservedForFutureIssuanceAbstract" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:stringItemType"/>
  <xs:element id="prax_UnauditedInterimCondensedConsolidatedFinancialInformationPolicyTextBlock" abstract="false" name="UnauditedInterimCondensedConsolidatedFinancialInformationPolicyTextBlock" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="nonnum:textBlockItemType"/>
  <xs:element id="prax_TemporaryEquityValueAcquired" abstract="false" name="TemporaryEquityValueAcquired" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="prax_PercentageIncreaseInAnnualBaseRent" abstract="false" name="PercentageIncreaseInAnnualBaseRent" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="num:percentItemType"/>
  <xs:element id="prax_AccruedExternalResearchAndDevelopmentExpensesCurrent" abstract="false" name="AccruedExternalResearchAndDevelopmentExpensesCurrent" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="prax_TemporaryEquitySharesAcquired" abstract="false" name="TemporaryEquitySharesAcquired" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:sharesItemType"/>
  <xs:element id="prax_TwoThousandTwentyEmployeeStockPurchasePlanMember" abstract="true" name="TwoThousandTwentyEmployeeStockPurchasePlanMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="nonnum:domainItemType"/>
  <xs:element id="prax_RogConIncMember" abstract="true" name="RogConIncMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="nonnum:domainItemType"/>
</xs:schema>
