<SUBMISSION>
<ACCESSION-NUMBER>0001140361-09-024221
<TYPE>3
<PUBLIC-DOCUMENT-COUNT>1
<PERIOD>20090828
<FILING-DATE>20091102
<DATE-OF-FILING-DATE-CHANGE>20091102
<REPORTING-OWNER>
<OWNER-DATA>
<CONFORMED-NAME>Park Mungo
<CIK>0001474363
</OWNER-DATA>
<FILING-VALUES>
<FORM-TYPE>3
<ACT>34
<FILE-NUMBER>001-33710
<FILM-NUMBER>091150002
</FILING-VALUES>
<MAIL-ADDRESS>
<STREET1>C/O CLEAN DIESEL TECHNOLOGIES, INC.
<STREET2>10 MIDDLE ST., STE 1100
<CITY>STAMFORD
<STATE>CT
<ZIP>06604
</MAIL-ADDRESS>
</REPORTING-OWNER>
<ISSUER>
<COMPANY-DATA>
<CONFORMED-NAME>CLEAN DIESEL TECHNOLOGIES INC
<CIK>0000949428
<ASSIGNED-SIC>2810
<IRS-NUMBER>061393453
<STATE-OF-INCORPORATION>DE
<FISCAL-YEAR-END>1231
</COMPANY-DATA>
<BUSINESS-ADDRESS>
<STREET1>10 MIDDLE STREET
<STREET2>SUITE 1100
<CITY>BRIDGEPORT
<STATE>CT
<ZIP>06604
<PHONE>2034165290
</BUSINESS-ADDRESS>
<MAIL-ADDRESS>
<STREET1>10 MIDDLE STREET
<STREET2>SUITE 1100
<CITY>BRIDGEPORT
<STATE>CT
<ZIP>06604
</MAIL-ADDRESS>
</ISSUER>
<DOCUMENT>
<TYPE>3
<SEQUENCE>1
<FILENAME>doc1.xml
<DESCRIPTION>FORM 3
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0203</schemaVersion>

    <documentType>3</documentType>

    <periodOfReport>2009-08-28</periodOfReport>

    <noSecuritiesOwned>0</noSecuritiesOwned>

    <issuer>
        <issuerCik>0000949428</issuerCik>
        <issuerName>CLEAN DIESEL TECHNOLOGIES INC</issuerName>
        <issuerTradingSymbol>CDTI</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001474363</rptOwnerCik>
            <rptOwnerName>Park Mungo</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O CLEAN DIESEL TECHNOLOGIES, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>10 MIDDLE ST., STE. 1100</rptOwnerStreet2>
            <rptOwnerCity>BRIDGEPORT</rptOwnerCity>
            <rptOwnerState>CT</rptOwnerState>
            <rptOwnerZipCode>06604</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable></nonDerivativeTable>

    <derivativeTable>
        <derivativeHolding>
            <securityTitle>
                <value>Warrant</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>12.50</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2007-07-02</value>
            </exerciseDate>
            <expirationDate>
                <value>2012-07-02</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>70255</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>Held by Affiliate of Reporting Person</value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Warrant</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>15.625</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2007-12-29</value>
            </exerciseDate>
            <expirationDate>
                <value>2012-12-29</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>73177</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>Held by Affiliate of Reporting Person</value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Warrant</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>8.4375</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2006-12-29</value>
            </exerciseDate>
            <expirationDate>
                <value>2011-12-29</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>140542</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>Held by Affiliate of Reporting Person</value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeHolding>
    </derivativeTable>

    <footnotes></footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Mungo Park</signatureName>
        <signatureDate>2009-11-02</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SUBMISSION>
