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

    <schemaVersion>X0206</schemaVersion>

    <documentType>3</documentType>

    <periodOfReport>2019-05-16</periodOfReport>

    <noSecuritiesOwned>0</noSecuritiesOwned>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001508061</rptOwnerCik>
            <rptOwnerName>OATV II, L.P.</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O OATV</rptOwnerStreet1>
            <rptOwnerStreet2>775 E. BLITHEDALE AVENUE, #568</rptOwnerStreet2>
            <rptOwnerCity>MILL VALLEY</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>94941</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001776845</rptOwnerCik>
            <rptOwnerName>OATVIISPV1, LLC</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O OATV</rptOwnerStreet1>
            <rptOwnerStreet2>775 E. BLITHEDALE AVENUE, #568</rptOwnerStreet2>
            <rptOwnerCity>MILL VALLEY</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>94941</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001776846</rptOwnerCik>
            <rptOwnerName>OATVIISPV2, LLC</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O OATV</rptOwnerStreet1>
            <rptOwnerStreet2>775 E. BLITHEDALE AVENUE, #568</rptOwnerStreet2>
            <rptOwnerCity>MILL VALLEY</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>94941</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001776794</rptOwnerCik>
            <rptOwnerName>O'Reilly AlphaTech Ventures II, LLC</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O OATV</rptOwnerStreet1>
            <rptOwnerStreet2>775 E. BLITHEDALE AVENUE, #568</rptOwnerStreet2>
            <rptOwnerCity>MILL VALLEY</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>94941</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001777004</rptOwnerCik>
            <rptOwnerName>Roberts Bryce</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O OATV</rptOwnerStreet1>
            <rptOwnerStreet2>775 E. BLITHEDALE AVENUE, #568</rptOwnerStreet2>
            <rptOwnerCity>MILL VALLEY</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>94941</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001776830</rptOwnerCik>
            <rptOwnerName>Jacobsen Mark</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O OATV</rptOwnerStreet1>
            <rptOwnerStreet2>775 E. BLITHEDALE AVENUE, #568</rptOwnerStreet2>
            <rptOwnerCity>MILL VALLEY</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>94941</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <derivativeTable>
        <derivativeHolding>
            <securityTitle>
                <value>Class B Common Stock</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F1"/>
            </conversionOrExercisePrice>
            <exerciseDate>
                <footnoteId id="F1"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F1"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Class A Common Stock</value>
                    <footnoteId id="F1"/>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>315287</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See Footnote</value>
                    <footnoteId id="F2"/>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Series Seed Convertible Preferred Stock</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F3"/>
            </conversionOrExercisePrice>
            <exerciseDate>
                <footnoteId id="F3"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F3"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Class B Common Stock</value>
                    <footnoteId id="F1"/>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>4024682</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See Footnote</value>
                    <footnoteId id="F2"/>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Series A Convertible Preferred Stock</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F4"/>
            </conversionOrExercisePrice>
            <exerciseDate>
                <footnoteId id="F4"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F4"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Class B Common Stock</value>
                    <footnoteId id="F1"/>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>1301677</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See Footnote</value>
                    <footnoteId id="F2"/>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Series B Convertible Preferred Stock</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F5"/>
            </conversionOrExercisePrice>
            <exerciseDate>
                <footnoteId id="F5"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F5"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Class B Common Stock</value>
                    <footnoteId id="F1"/>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>1419182</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See Footnote</value>
                    <footnoteId id="F2"/>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Series C Convertible Preferred Stock</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F6"/>
            </conversionOrExercisePrice>
            <exerciseDate>
                <footnoteId id="F6"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F6"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Class B Common Stock</value>
                    <footnoteId id="F1"/>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>845507</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See Footnote</value>
                    <footnoteId id="F2"/>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Series D Convertible Preferred Stock</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F7"/>
            </conversionOrExercisePrice>
            <exerciseDate>
                <footnoteId id="F7"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F7"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Class B Common Stock</value>
                    <footnoteId id="F1"/>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>533752</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See Footnote</value>
                    <footnoteId id="F8"/>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Series D Convertible Preferred Stock</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F7"/>
            </conversionOrExercisePrice>
            <exerciseDate>
                <footnoteId id="F7"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F7"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Class B Common Stock</value>
                    <footnoteId id="F1"/>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>38837</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See Footnote</value>
                    <footnoteId id="F9"/>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Series E Convertible Preferred Stock</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F10"/>
            </conversionOrExercisePrice>
            <exerciseDate>
                <footnoteId id="F10"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F10"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Class B Common Stock</value>
                    <footnoteId id="F1"/>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>49567</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See Footnote</value>
                    <footnoteId id="F9"/>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeHolding>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">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. Following the closing of the initial public offering, 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>
        <footnote id="F2">These shares are held directly by OATV II, L.P. (&quot;OATV II&quot;). The general partner is O'Reilly AlphaTech Ventures II, LLC (&quot;O'Reilly AlphaTech&quot;). The individual managing members of O'Reilly AlphaTech are Bryce Roberts and Mark Jacobsen, and they may be deemed to share voting and dispositive power over the shares held by OATV II. Such persons and entities disclaim beneficial ownership over shares held by OATV II, except to the extent of any pecuniary interest therein.</footnote>
        <footnote id="F3">The Series Seed Convertible Preferred Stock is convertible into Class B Common Stock on a 1:1 basis and has no expiration date. Immediately upon the closing of the Issuer's initial public offering, all shares of Series Seed Convertible Preferred Stock will be automatically converted into shares of Class B Common Stock.</footnote>
        <footnote id="F4">The Series A Convertible Preferred Stock is convertible into Class B Common Stock on a 1:1 basis and has no expiration date. Immediately upon the closing of the Issuer's initial public offering, all shares of Series A Convertible Preferred Stock will be automatically converted into shares of Class B Common Stock.</footnote>
        <footnote id="F5">The Series B Convertible Preferred Stock is convertible into Class B Common Stock on a 1:1 basis and has no expiration date. Immediately upon the closing of the Issuer's initial public offering, all shares of Series B Convertible Preferred Stock will be automatically converted into shares of Class B Common Stock.</footnote>
        <footnote id="F6">The Series C Convertible Preferred Stock is convertible into Class B Common Stock on a 1:1 basis and has no expiration date. Immediately upon the closing of the Issuer's initial public offering, all shares of Series C Convertible Preferred Stock will be automatically converted into shares of Class B Common Stock.</footnote>
        <footnote id="F7">The Series D Convertible Preferred Stock is convertible into Class B Common Stock on a 1:1 basis and has no expiration date. Immediately upon the closing of the Issuer's initial public offering, all shares of Series D Convertible Preferred Stock will be automatically converted into shares of Class B Common Stock.</footnote>
        <footnote id="F8">These shares are held directly by OATVIISPV1, LLC (&quot;OATVIISPV1&quot;). The general partner is O'Reilly AlphaTech Ventures II, LLC (&quot;O'Reilly AlphaTech&quot;). The individual managing members of O'Reilly AlphaTech are Bryce Roberts and Mark Jacobsen, and they may be deemed to share voting and dispositive power over the shares held by OATVIISPV1. Such persons and entities disclaim beneficial ownership over shares held by OATVIISPV1, except to the extent of any pecuniary interest therein.</footnote>
        <footnote id="F9">These shares are held directly by OATVIISPV2, LLC (&quot;OATVIISPV2&quot;). The general partner is O'Reilly AlphaTech Ventures II, LLC (&quot;O'Reilly AlphaTech&quot;). The individual managing members of O'Reilly AlphaTech are Bryce Roberts and Mark Jacobsen, and they may be deemed to share voting and dispositive power over the shares held by OATVIISPV2. Such persons and entities disclaim beneficial ownership over shares held by OATVIISPV2, except to the extent of any pecuniary interest therein.</footnote>
        <footnote id="F10">The Series E Convertible Preferred Stock is convertible into Class B Common Stock on a 1:1 basis and has no expiration date. Immediately upon the closing of the Issuer's initial public offering, all shares of Series E Convertible Preferred Stock will be automatically converted into shares of Class B Common Stock.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>OATV II, L.P., By: O'Reilly AlphaTech Ventures II, LLC, its general partner, By: /s/ Laura Kelley, Chief Financial Officer</signatureName>
        <signatureDate>2019-05-16</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>OATVIISPV1, LLC, By: O'Reilly AlphaTech Ventures II, LLC, its general partner, By: /s/ Laura Kelley, Chief Financial Officer</signatureName>
        <signatureDate>2019-05-16</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>OATVIISPV2, LLC, By: O'Reilly AlphaTech Ventures II, LLC, its general partner, By: /s/ Laura Kelley, Chief Financial Officer</signatureName>
        <signatureDate>2019-05-16</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>O'Reilly AlphaTech Ventures II, LLC, By: /s/ Laura Kelley, Chief Financial Officer</signatureName>
        <signatureDate>2019-05-16</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>/s/ Bryce Roberts</signatureName>
        <signatureDate>2019-05-16</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>/s/ Mark Jacobsen</signatureName>
        <signatureDate>2019-05-16</signatureDate>
    </ownerSignature>
</ownershipDocument>
