Exhibit 5.1

Seth J. Gottlieb

+1 650 843 5864

sgottlieb@cooley.com

October 15, 2020

Fastly, Inc.

475 Brannan Street, Suite 300

San Francisco, CA 94107

Ladies and Gentlemen:

You have requested our opinion, as counsel to Fastly, Inc., a Delaware corporation (the “Company"), with respect to certain matters in connection with the filing by the Company of a Registration Statement on Form S-3 (the “Registration Statement”)with the Securities and Exchange Commission, including a related prospectus (the “Prospectus”), covering the registration for resale of 6,367,709 shares of Class A common stock, $0.00002 par value, of the Company (the “Shares”). We have been advised that the Shares have been issued by the Company pursuant to that certain Agreement and Plan of Reorganization, dated as of August 26, 2020, by and among the Company, One Fish, Inc., Two Fish, LLC, Signal Sciences Corp. and Shareholder Representative Services, LLC (the “Merger Agreement”).

In connection with this opinion, we have examined and relied upon the Registration Statement, the Prospectus, the Company's Amended and Restated Certificate of Incorporation, as amended, and Amended and Restated Bylaws, the Merger Agreement and originals or copies certified to our satisfaction of such records, documents, certificates, memoranda and other instruments as in our judgment are necessary or appropriate to enable us to render the opinion expressed below. We have assumed the genuineness of all signatures, the authenticity of all documents submitted to us as originals, the conformity to originals of all documents submitted to us as copies; the accuracy, completeness and authenticity of certificates of public officials, and the due authorization, execution and delivery of all documents by all persons other than the Company where authorization, execution and delivery are prerequisites to the effectiveness thereof. As to certain factual matters, we have relied upon a certificate of an officer of the Company and have not independently verified such matters.

Our opinion is expressed only with respect to the General Corporation Law of the State of Delaware. We express no opinion to the extent that any other laws are applicable to the subject matter hereof and express no opinion and provide no assurance as to compliance with any federal or state securities law, rule or regulation.

On the basis of the foregoing, and in reliance thereon, we are of the opinion that the Shares have been validly issued and are fully paid and non-assessable.

Cooley LLP 3175 Hanover Street Palo Alto. CA 94304 -1130

t: (650) 843-5000 f: (650) 849-7400 cooley.com

October 15, 2020

Page Two

We consent to the reference to our firm under the caption “Legal Matters” in the Prospectus and to the filing of this opinion as an exhibit to the Registration Statement.

Sincerely,

Cooley LLP

By:/s/ Seth J. Gottlieb    

Seth J. Gottlieb

Cooley LLP 3175 Hanover Street Palo Alto, CA 94304-1130

t: (650) 843-5000 f: (650) 849-7400 cooley.com