<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2022-02-25</periodOfReport>

    <issuer>
        <issuerCik>0001464423</issuerCik>
        <issuerName>PennyMac Mortgage Investment Trust</issuerName>
        <issuerTradingSymbol>PMT</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001486360</rptOwnerCik>
            <rptOwnerName>Fartaj Vandad</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O PENNYMAC MORTGAGE INVESTMENT TRUST</rptOwnerStreet1>
            <rptOwnerStreet2>3043 TOWNSGATE ROAD</rptOwnerStreet2>
            <rptOwnerCity>WESTLAKE VILLAGE</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>91361</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isOfficer>1</isOfficer>
            <officerTitle>Chief Investment Officer</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Shares of Beneficial Interest</value>
            </securityTitle>
            <transactionDate>
                <value>2022-02-25</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>14367</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>125598</value>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">The Reporting Person was granted 14,367 restricted share units. The restricted share units, which vest in three equal installments beginning on the first anniversary of the grant date, are to be settled in an equal number of shares of beneficial interest upon vesting.</footnote>
        <footnote id="F2">The reported amount consists of 26,412 restricted share units and 99,186 common shares of beneficial interest. The restricted share units are to be settled in an equal number of common shares of beneficial interest upon vesting.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Derek W. Stark, attorney-in-fact for Mr. Fartaj</signatureName>
        <signatureDate>2022-03-01</signatureDate>
    </ownerSignature>
</ownershipDocument>
