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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2012-11-02</periodOfReport>

    <notSubjectToSection16>1</notSubjectToSection16>

    <issuer>
        <issuerCik>0001459450</issuerCik>
        <issuerName>Myrexis, Inc.</issuerName>
        <issuerTradingSymbol>MYRX</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001325447</rptOwnerCik>
            <rptOwnerName>First Eagle Investment Management, LLC</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>1345 AVENUE OF THE AMERICAS</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10105</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock, $.01 par value (the &quot;Common Stock&quot;)</value>
            </securityTitle>
            <transactionDate>
                <value>2012-11-02</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>42000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>2.38</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>2671617</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See footnote</value>
                    <footnoteId id="F1"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">The shares of Common Stock reported herein are indirectly beneficially owned by First Eagle Investment Management, LLC, a Delaware limited liability company and an investment adviser registered under the Investment Advisers Act of 1940 (&quot;FEIM&quot;). FEIM is a subsidiary of Arnhold and S. Bleichroeder Holdings, Inc. The shares are held by various clients in accounts under FEIM's management and control. FEIM disclaims beneficial ownership of the shares of Common Stock as provided in Rule 16a-1 and otherwise to the extent such beneficial ownership exceeds its pecuniary interest therein.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>Mark Goldstein</signatureName>
        <signatureDate>2012-11-05</signatureDate>
    </ownerSignature>
</ownershipDocument>
