-----BEGIN PRIVACY-ENHANCED MESSAGE-----
Proc-Type: 2001,MIC-CLEAR
Originator-Name: webmaster@www.sec.gov
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MIC-Info: RSA-MD5,RSA,
 EBiWrdQu+u1uE8s021qtBg4OyPaLChSiEeQdPC0wNuTnrfAkWfXjaHGM4QWgyfJZ
 0FSbYwIfAmVufYadNGzXNg==

<SEC-DOCUMENT>0000040888-05-000005.txt : 20050118
<SEC-HEADER>0000040888-05-000005.hdr.sgml : 20050117
<ACCEPTANCE-DATETIME>20050118120300
ACCESSION NUMBER:		0000040888-05-000005
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20050115
FILED AS OF DATE:		20050118
DATE AS OF CHANGE:		20050118

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			OSTERHOFF JAMES M
		CENTRAL INDEX KEY:			0001212523

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	001-01520
		FILM NUMBER:		05532792

	BUSINESS ADDRESS:	
		BUSINESS PHONE:		9163518515

	MAIL ADDRESS:	
		STREET 1:		C/O GENCORP INC.,
		STREET 2:		P.O. BOX 537012
		CITY:			SACRAMENTO
		STATE:			CA
		ZIP:			95853-7012

ISSUER:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			GENCORP INC
		CENTRAL INDEX KEY:			0000040888
		STANDARD INDUSTRIAL CLASSIFICATION:	FABRICATED RUBBER PRODUCTS, NEC [3060]
		IRS NUMBER:				340244000
		STATE OF INCORPORATION:			OH
		FISCAL YEAR END:			1130

	BUSINESS ADDRESS:	
		STREET 1:		HIGHWAY 50 & AEROJET ROAD
		CITY:			RANCHO CORDOVA
		STATE:			CA
		ZIP:			95670
		BUSINESS PHONE:		9163554000

	MAIL ADDRESS:	
		STREET 1:		HIGHWAY 50 & AEROJET ROAD
		CITY:			RANCHO CORDOVA
		STATE:			CA
		ZIP:			95670

	FORMER COMPANY:	
		FORMER CONFORMED NAME:	GENERAL TIRE & RUBBER CO
		DATE OF NAME CHANGE:	19840330
</SEC-HEADER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>doc.xml
<DESCRIPTION>PRIMARY DOCUMENT
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0202</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2005-01-15</periodOfReport>

    <issuer>
        <issuerCik>0000040888</issuerCik>
        <issuerName>GENCORP INC</issuerName>
        <issuerTradingSymbol>GY</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001212523</rptOwnerCik>
            <rptOwnerName>OSTERHOFF JAMES M</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>HIGHWAY 50 &amp; AEROJET ROAD</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>RANCHO CORDOVA</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>95670</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>
        <derivativeTransaction>
            <securityTitle>
                <value>Phantom Stock   Unit</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>0</value>
                <footnoteId id="F1"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2005-01-15</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F2"/>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>303.197</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>18.14</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F3"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F4"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>303.197</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>20000.952</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>By Plan</value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F2">Phantom Stock Units acquired under the Deferred Compensation Plan for Nonemployee Directors on 1-15-05.</footnote>
        <footnote id="F4">These Derivative Securities are Phantom Stock Units in the GenCorp Deferred Compensation Plan for Nonemployee Directors.  The Phantom Stock Units are payable in cash upon a future date elected by the Director at the time of deferral, based upon the market value of GenCorp common shares at the time of payment.  Therefore there is no Expiration Date for these Derivative Securities.</footnote>
        <footnote id="F1">1 for 1; one phantom share equals one share of GenCorp common stock.</footnote>
        <footnote id="F3">These Derivative Securities are Phantom Stock Units in the GenCorp Deferred Compensation Plan for Nonemployee Directors.  The Phantom Stock Units are payable in cash upon a future date elected by the Director at the time of deferral, based upon the market value of GenCorp common shares at the time of payment.  Therefore there is no Date Exercisable for these Derivative Securities.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>Mark A. Whitney as Attorney-in-Fact</signatureName>
        <signatureDate>2005-01-18</signatureDate>
    </ownerSignature>
</ownershipDocument>

</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
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