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

    <schemaVersion>X0205</schemaVersion>

    <documentType>3</documentType>

    <periodOfReport>2012-05-29</periodOfReport>

    <noSecuritiesOwned>0</noSecuritiesOwned>

    <issuer>
        <issuerCik>0001320947</issuerCik>
        <issuerName>Diamond Foods Inc</issuerName>
        <issuerTradingSymbol>DMND</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001550356</rptOwnerCik>
            <rptOwnerName>Wilson Matthew C.</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O DIAMOND FOODS, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>600 MONTGOMERY STREET, 13TH FLOOR</rptOwnerStreet2>
            <rptOwnerCity>SAN FRANCISCO</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>94111</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable></nonDerivativeTable>

    <derivativeTable>
        <derivativeHolding>
            <securityTitle>
                <value>Common Stock Purchase Warrant (right to buy)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>10</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <footnoteId id="F1"/>
            </exerciseDate>
            <expirationDate>
                <value>2019-05-29</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>4420859</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>by OCM PF/FF Adamantine Holdings, Ltd.</value>
                    <footnoteId id="F2"/>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeHolding>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">The warrant will only be exercisable after the earlier of (i) the date when there are less than $75 million of the Senior Notes outstanding (as defined in that certain Securities Purchase Agreement entered into by and between the Issuer and OCM PF/FF Adamantine Holdings, Ltd. (&quot;OCM&quot;) on May 22, 2012 ( the &quot;Purchase Agreement&quot;)), (ii) March 1, 2013, provided the Issuer has not taken certain actions towards the cancellation of the warrant pursuant to the Purchase Agreement prior to such date, (iii) the date of a merger, stock sale, asset sale consisting of substantially all of the Issuer's assets, or similar transaction; the dissolution, liquidation, winding up, reorganization, reclassification, or recapitalization of the Issuer, and (iv) May 1, 2013.</footnote>
        <footnote id="F2">Oaktree Capital Management, L.P. (&quot;Oaktree&quot;) is the director of OCM.  The Reporting Person is a managing director of Oaktree who serves as a director of the Issuer appointed on behalf of OCM.  The warrant may be deemed beneficially owned by the Reporting Person by virtue of his being an officer of OCM, and the Reporting Person disclaims beneficial ownership of the warrant except to the extent of his indirect pecuniary interest therein.  The inclusion of these securities in this Form 3 shall not be deemed an admission that the Reporting Person has beneficial ownership of such warrant for purposes of Section 16 of the Securities Exchange Act of 1934, or for any other purpose.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Matthew C. Wilson by Stephen Kim, Attorney-in-Fact</signatureName>
        <signatureDate>2012-05-31</signatureDate>
    </ownerSignature>
</ownershipDocument>
