EX-5.1

Exhibit 5.1

LOGO

JOHN T. MCKENNA

+1 650 843 5059

jmckenna@cooley.com

March 28, 2019

Forty Seven, Inc.

1490 O’Brien Drive, Suite A

Menlo Park, CA 94025

Ladies and Gentlemen:

We have acted as counsel to Forty Seven, 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 an aggregate of 1,864,749 shares of the Company’s Common Stock, par value $0.0001 per share, consisting of (i) 1,553,957 shares (the “EIP Shares”) pursuant to the Company’s 2018 Equity Incentive Plan, (the “2018 EIP”), and (ii) 310,792 shares (the “ESPP Shares,” together with the EIP Shares, the “Shares”) pursuant to the Company’s 2018 Employee Stock Purchase Plan (the “2018 ESPP,” and together with the 2018 EIP, the “Plans”).

In connection with this opinion, we have examined and relied upon (a) the Registration Statement and related prospectuses, (b) the Plans, (c) the Company’s Amended and Restated Certificate of Incorporation and Amended and Restated Bylaws, each as currently in effect, and (d) such other documents, records, 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, and the conformity to originals of all documents submitted to us as copies 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 and 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).

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Cooley LLP

By:		/s/ John T. McKenna
		John T. McKenna