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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2014-11-28</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000892986</issuerCik>
        <issuerName>WARREN RESOURCES INC</issuerName>
        <issuerTradingSymbol>WRES</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001564285</rptOwnerCik>
            <rptOwnerName>EPSTEIN PHILIP ARTHUR</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>1114 AVENUE OF THE AMERICAS</rptOwnerStreet1>
            <rptOwnerStreet2>34TH FLOOR</rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10036</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Chief Executive Officer</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2014-11-28</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>P</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>10000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>2.22</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>1133852</value>
                    <footnoteId id="F1"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>By IRA Account</value>
                    <footnoteId id="F2"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">Includes 772,222 Restricted Shares of Common Stock that vest, if at all, based on the average closing trading price (&quot;ACTP&quot;) of Warren Resources, Inc.'s Common Stock over any period of (30) consecutive trading days (a &quot;Thirty Day Period&quot;) occurring during the period commencing December 5, 2012 with respect to 386,111 Restricted Shares and on January 2, 2013 with respect to another 386,111 Restricted Shares (collectively, the &quot;Effective Date&quot;) and ending on the last business day occurring before the fifth anniversary of the Effective Dates (the &quot;Performance Period&quot;) as follows: 193,056 Restricted Shares on the first day that the ACTP for any Thirty Day Period equals or exceeds any higher whole dollar increment ranging from $7 to $10</footnote>
        <footnote id="F2">The additional 10,000 shares acquired on November 28, 2014 were purchased by the Reporting Person's IRA account.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>Philip A. Epstein</signatureName>
        <signatureDate>2014-12-01</signatureDate>
    </ownerSignature>
</ownershipDocument>
