EX-5.1

Exhibit 5.1

LOGO

Seth J. Gottlieb

+1 650 843 5864

sgottlieb@cooley.com

April 13, 2020

Fastly, Inc.

475 Brannan Street, Suite 300

San Francisco, CA 94107

Ladies and Gentlemen:

We have acted as counsel to Fastly, Inc., a Delaware corporation (the “Company”), in connection with the filing of a registration statement on Form S-8 (the “Registration Statement”) with the Securities and Exchange Commission covering the offering of up to (a) 4,750,880 shares of Class A common stock, par value $0.00002 per share, (the “2019 EIP Shares”) issuable pursuant to the Company’s 2019 Equity Incentive Plan (the “2019 EIP”) and (b) 950,176 shares of Class A common stock, par value $0.00002 per share (together with the 2019 EIP Shares, the “Shares”), issuable pursuant to the Company’s 2019 Employee Stock Purchase Plan (together with the Company’s 2019 EIP, the “Plans”).

In connection with this opinion, we have examined and relied upon (a) the Registration Statement and related prospectuses, (b) the Company’s Amended and Restated Certificate of Incorporation and Amended and Restated Bylaws, each as currently in effect, (c) the Plans, and (d) 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 and authenticity of all documents submitted to us as originals, the conformity to originals of all documents submitted to us as copies thereof, 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 due 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 sought independently to verify 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, when sold and issued in accordance with the Plans, the Registration Statement and related prospectuses, will be validly issued, fully paid, and nonassessable (except as to shares issued pursuant to certain deferred payment arrangements, which will be fully paid and nonassessable when such deferred payments are made in full).

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

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