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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2014-02-06</periodOfReport>

    <issuer>
        <issuerCik>0000746838</issuerCik>
        <issuerName>UNISYS CORP</issuerName>
        <issuerTradingSymbol>UIS</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001185122</rptOwnerCik>
            <rptOwnerName>FLETCHER DENISE K</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>801 LAKEVIEW DRIVE, SUITE 100</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>BLUE BELL</rptOwnerCity>
            <rptOwnerState>PA</rptOwnerState>
            <rptOwnerZipCode>19422</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2014-02-06</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>4746</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>38717.3</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">Restricted stock units granted under the terms and provisions of the Unisys Corporation 2003 Long-Term Incentive and Equity Compensation Plan. The stock units vested 100% on the date of grant, February 6, 2014.  The stock units have been deferred under the terms and provisions of the 2005 Deferred Compensation Plan for Directors of Unisys Corporation.  Deferred units will be distributed in shares of Unisys common stock either upon the earlier of termination of service or on any date at least two years after the stock units are awarded, as previously elected by the director.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>Susan T. Keene, attorney-in-fact, for Denise K. Fletcher</signatureName>
        <signatureDate>2014-02-10</signatureDate>
    </ownerSignature>
</ownershipDocument>
