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<ownershipDocument>

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2019-11-25</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001517413</issuerCik>
        <issuerName>Fastly, Inc.</issuerName>
        <issuerTradingSymbol>FSLY</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001775868</rptOwnerCik>
            <rptOwnerName>Amplify GP Partners, LLC</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>800 MENLO AVENUE</rptOwnerStreet1>
            <rptOwnerStreet2>SUITE 220</rptOwnerStreet2>
            <rptOwnerCity>MENLO PARK</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>94025</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001562623</rptOwnerCik>
            <rptOwnerName>Amplify Partners, L.P.</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>800 MENLO AVENUE</rptOwnerStreet1>
            <rptOwnerStreet2>SUITE 220</rptOwnerStreet2>
            <rptOwnerCity>MENLO PARK</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>94025</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001775996</rptOwnerCik>
            <rptOwnerName>AP Opportunity Fund LLC</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>800 MENLO AVENUE</rptOwnerStreet1>
            <rptOwnerStreet2>SUITE 220</rptOwnerStreet2>
            <rptOwnerCity>MENLO PARK</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>94025</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Class A Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2019-11-25</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
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                <transactionCode>C</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
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            <transactionTimeliness>
                <value></value>
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            <transactionAmounts>
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                    <value>1200000</value>
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            <ownershipNature>
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                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See Footnote</value>
                    <footnoteId id="F2"/>
                </natureOfOwnership>
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        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Class A Common Stock</value>
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            <transactionDate>
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            <deemedExecutionDate></deemedExecutionDate>
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                </sharesOwnedFollowingTransaction>
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            <ownershipNature>
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                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See Footnote</value>
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        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Class A Common Stock</value>
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            <transactionDate>
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            <deemedExecutionDate></deemedExecutionDate>
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            <transactionTimeliness>
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            <transactionAmounts>
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                <natureOfOwnership>
                    <value>See Footnote</value>
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        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Class A Common Stock</value>
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            <transactionDate>
                <value>2019-11-25</value>
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            <deemedExecutionDate></deemedExecutionDate>
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            <transactionTimeliness>
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            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See Footnote</value>
                    <footnoteId id="F3"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Class A Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2019-11-25</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
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                <footnoteId id="F4"/>
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                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
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                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See Footnote</value>
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                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Class A Common Stock</value>
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            <transactionDate>
                <value>2019-11-25</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
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            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See Footnote</value>
                    <footnoteId id="F5"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
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    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Class B Common Stock</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F6"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2019-11-25</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>C</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
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            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>1200000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0.00</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F6"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F6"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Class A Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>1200000</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>5298407</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See Footnote</value>
                    <footnoteId id="F2"/>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeTransaction>
        <derivativeTransaction>
            <securityTitle>
                <value>Class B Common Stock</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F6"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2019-11-25</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>C</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
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            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>325072</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0.00</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F6"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F6"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Class A Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>325072</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>1435298</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See Footnote</value>
                    <footnoteId id="F3"/>
                </natureOfOwnership>
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        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Each share of Class B Common Stock converted into Class A Common Stock upon the election of the reporting person.</footnote>
        <footnote id="F2">These shares are held directly by Amplify Partners, L.P. The sole general partner is Amplify GP Partners, LLC. Sunil Dhaliwal, a member of the Issuer's Board of Directors, is a managing member of Amplify GP Partners, LLC, and may be deemed to have voting and investment power with respect to these securities. Amplify GP Partners, LLC and Sunil Dhaliwal disclaims beneficial ownership of the securities except to the extent of his or its pecuniary interest therein.</footnote>
        <footnote id="F3">These shares are held directly by AP Opportunity Fund LLC. The sole general partner is Amplify GP Partners, LLC. Sunil Dhaliwal, a member of the Issuer's Board of Directors, is a managing member of Amplify GP Partners, LLC, and may be deemed to have voting and investment power with respect to these securities. Amplify GP Partners, LLC and Sunil Dhaliwal disclaims beneficial ownership of the securities except to the extent of his or its pecuniary interest therein.</footnote>
        <footnote id="F4">The reporting person distributed, for no consideration, the shares of Class A Common Stock to its partners, representing each such partner's pro rata interest in such shares.</footnote>
        <footnote id="F5">These shares are held directly by Amplify GP Partners, LLC. Sunil Dhaliwal, a member of the Issuer's Board of Directors, is a managing member of Amplify GP Partners, LLC, and may be deemed to have voting and investment power with respect to these securities. Amplify GP Partners, LLC and Sunil Dhaliwal disclaims beneficial ownership of the securities except to the extent of his or its pecuniary interest therein.</footnote>
        <footnote id="F6">Each share of Class B Common Stock will convert automatically into Class A Common Stock upon any transfer except certain &quot;Permitted Transfers&quot; described in the Issuer's amended and restated certificate of incorporation. All outstanding shares of Class B Common Stock will convert into shares of Class A Common Stock upon the earliest of (i) the first trading day falling nine months after the date on which the shares of Class B Common Stock represent less than 10% of the aggregate number of outstanding shares of Class A Common Stock and Class B Common Stock, (ii) the date specified by the vote of the holders of Class B Common Stock representing at least a majority of the outstanding shares of Class B Common Stock, or (iii) the first trading day falling ten years after the Effective Time as defined in the Issuer's amended and restated certificate of incorporation.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Amplify GP Partners, LLC, By: /s/ Steve Agelopoulos, Chief Financial Officer</signatureName>
        <signatureDate>2019-11-27</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>Amplify Partners, L.P., By: Amplify GP Partners, LLC, its sole general partner, By: /s/ Steve Agelopoulos, Chief Financial Officer</signatureName>
        <signatureDate>2019-11-27</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>AP Opportunity Fund LLC, By: Amplify GP Partners, LLC, its sole general partner, By: /s/ Steve Agelopoulos, Chief Financial Officer</signatureName>
        <signatureDate>2019-11-27</signatureDate>
    </ownerSignature>
</ownershipDocument>
