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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2020-05-26</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001082038</issuerCik>
        <issuerName>DURECT CORP</issuerName>
        <issuerTradingSymbol>DRRX</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001781002</rptOwnerCik>
            <rptOwnerName>Bleichroeder LP</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>1345 AVENUE OF THE AMERICAS</rptOwnerStreet1>
            <rptOwnerStreet2>47TH FLOOR</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</value>
            </securityTitle>
            <transactionDate>
                <value>2020-05-26</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>39388</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>2.5014</value>
                    <footnoteId id="F2"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>26664656</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See Footnote</value>
                    <footnoteId id="F3"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">The transaction reported herein was entered into in error. In addition, the sale by the reporting person of 39,388 shares of Issuer's common stock reported herein was matchable under Section 16(b) of the Securities Exchange Act of 1934 with the purchase by the reporting person of 39,388 shares of the Issuer's common stock on February 6, 2020 as reported in a prior Form 4 filing. Pursuant to a settlement agreement with the Issuer, the reporting person has paid to the issuer $34,588.00, representing the full amount of the profit realized in connection with the short-swing transaction.</footnote>
        <footnote id="F2">The reported price in Column 4 is a weighted average price. These shares were sold in multiple transactions. Prices ranged from $2.50 to $2.511 per share, inclusive. The Reporting Person undertakes to provide to the Issuer, any security holder of the Issuer, or to the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote.</footnote>
        <footnote id="F3">This form is filed by Bleichroeder LP. Bleichroeder LP serves as registered investment adviser to 21 April Fund, Ltd., 21 April Fund, LP and other managed accounts that directly hold these securities. Bleichroeder LP disclaims beneficial ownership of these securities except to the extent of their pecuniary interest therein, and this report shall not be deemed an admission that Bleichroeder LP is the beneficial owner of the securities for purposes of Section 16 or for any other purposes. 21 April Fund, Ltd. owns 16,036,514 shares and 21 April Fund, LP owns 4,718,700 shares respectively of these 26,664,656 shares reported in this form.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>Michael M. Kellen, Chairman and CO-CEO of Bleichroeder LP</signatureName>
        <signatureDate>2020-05-28</signatureDate>
    </ownerSignature>
</ownershipDocument>
