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

    <schemaVersion>X0206</schemaVersion>

    <documentType>3</documentType>

    <periodOfReport>2014-04-15</periodOfReport>

    <noSecuritiesOwned>0</noSecuritiesOwned>

    <issuer>
        <issuerCik>0001092839</issuerCik>
        <issuerName>DUNE ENERGY INC</issuerName>
        <issuerTradingSymbol>DUNR</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001562514</rptOwnerCik>
            <rptOwnerName>Zell Credit Opportunities Side Fund, L.P.</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>TWO NORTH RIVERSIDE PLAZA</rptOwnerStreet1>
            <rptOwnerStreet2>SUITE 600</rptOwnerStreet2>
            <rptOwnerCity>CHICAGO</rptOwnerCity>
            <rptOwnerState>IL</rptOwnerState>
            <rptOwnerZipCode>60606</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>1</isOther>
            <officerTitle></officerTitle>
            <otherText>Members of 10% Owner Group</otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001250731</rptOwnerCik>
            <rptOwnerName>CHAI TRUST CO LLC</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>TWO NORTH RIVERSIDE PLAZA</rptOwnerStreet1>
            <rptOwnerStreet2>SUITE 600</rptOwnerStreet2>
            <rptOwnerCity>CHICAGO</rptOwnerCity>
            <rptOwnerState>IL</rptOwnerState>
            <rptOwnerZipCode>60606</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>1</isOther>
            <officerTitle></officerTitle>
            <otherText>Members of 10% Owner Group</otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock, par value $0.001 per share</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>4637762</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                    <footnoteId id="F1"/>
                    <footnoteId id="F2"/>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">The shares of common stock, par value $0.001 per share (the &quot;Common Stock&quot;), of Dune Energy, Inc. (the &quot;Issuer&quot;) are held directly by Zell Credit Opportunities Side Fund, L.P., (the &quot;Side Fund&quot;). Chai Trust Company, LLC (&quot;Chai Trust&quot; and, together with the Side Fund, the &quot;Reporting Persons&quot;) is the sole general partner and investment manager of Side Fund and, in such capacity, may be deemed to beneficially own the 4,637,762 shares of Common Stock of the Issuer held directly by the Side Fund.</footnote>
        <footnote id="F2">As disclosed in the Schedule 13D/A filed by the Reporting Persons on April 18, 2014, the Reporting Persons and Whitebox Advisors, LLC may be deemed to be members of a &quot;group&quot; with respect to the Common Stock for purposes of Section 13(d) or Section 13(g) of the Securities Exchange Act of 1934, as amended (the &quot;Exchange Act&quot;). The Reporting Persons disclaim beneficial ownership of any shares of Common Stock not held directly by Side Fund or Chai Trust, and this report shall not be construed as an admission that the Reporting Persons are the beneficial owner of any shares of Common Stock held directly by Whitebox Advisors, LLC or any other shareholder for purposes of Section 16 of the Exchange Act or for any other purpose.</footnote>
    </footnotes>

    <remarks>Exhibit List: The Joint Filing Agreement filed as Exhibit 9 to the Schedule 13D/A filed by the Reporting Persons on April 18, 2014 is incorporated herein by reference.</remarks>

    <ownerSignature>
        <signatureName>Zell Credit Opportunities Side Fund, L.P., By: Chai Trust Company, LLC; Its: General Partner; By: /s/ Philip G. Tinkler, Chief Financial Officer</signatureName>
        <signatureDate>2014-04-25</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>Chai Trust Company, LLC; By: /s/ Philip G. Tinkler, Chief Financial Officer</signatureName>
        <signatureDate>2014-04-25</signatureDate>
    </ownerSignature>
</ownershipDocument>
