As filed with the U.S. Securities and Exchange Commission on June 27, 2018

Registration No. 333-            

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

FORM S-1

REGISTRATION STATEMENT

UNDER

THE SECURITIES ACT OF 1933

FORTY SEVEN, INC.

(Exact Name of Registrant as Specified in its Charter)

Forty Seven, Inc.

1490 O’Brien Drive, Suite A

Menlo Park, California 94025

(650) 352-4150

(Address, Including Zip Code, and Telephone Number, Including Area Code, of Registrant’s Principal Executive Offices)

Mark A. McCamish, M.D., Ph.D.

President and Chief Executive Officer

Forty Seven, Inc.

1490 O’Brien Drive, Suite A

Menlo Park, California 94025

(650) 352-4150

(Name, address, including zip code, and telephone number, including area code, of agent for service)

Copies to:

Approximate date of commencement of proposed sale to the public:

As soon as practicable after the effective date of this registration statement.

If any of the securities being registered on this Form are to be offered on a delayed or continuous basis pursuant to Rule 415 under the Securities Act of 1933 check the following box:  ☐

If this Form is filed to register additional securities for an offering pursuant to Rule 462(b) under the Securities Act, please check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.  ☑ (File No. 333-225390)

If this Form is a post-effective amendment filed pursuant to Rule 462(c) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.  ☐

If this Form is a post-effective amendment filed pursuant to Rule 462(d) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.  ☐

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, a smaller reporting company or an emerging growth company. See the definitions of “large accelerated filer,” “accelerated filer,” “smaller reporting company” and “emerging growth company” in Rule 12b-2 of the Exchange Act.

CALCULATION OF REGISTRATION FEE

This Registration Statement shall become effective upon filing in accordance with Rule 462(b) under the Securities Act of 1933, as amended.

EXPLANATORY NOTE AND INCORPORATION OF CERTAIN INFORMATION BY REFERENCE

Pursuant to Rule 462(b) under the Securities Act of 1933, as amended, Forty Seven, Inc. (the “Registrant”) is filing this Registration Statement on Form S-1 (this “Registration Statement”) with the Securities and Exchange Commission (the “Commission”). This Registration Statement relates to the public offering of securities contemplated by the Registration Statement on Form S-1, as amended (File No. 333-225390) (the “Prior Registration Statement”), which the Registrant originally filed with the Commission on June 1, 2018, and which the Commission declared effective on June 27, 2018.

The Registrant is filing this Registration Statement for the sole purpose of increasing the aggregate number of shares of common stock offered by the Registrant by 385,250 shares, 50,250 of which may be sold by the Registrant in the event the underwriters exercise their option to purchase additional shares of the Registrant’s common stock to cover over-allotments. The additional shares that are being registered for issuance and sale are in an amount and at a price that together represent no more than 20% of the maximum aggregate offering price set forth in the Calculation of Registration Fee table contained in the Prior Registration Statement. The information set forth in the Prior Registration Statement and all exhibits to the Prior Registration Statement are hereby incorporated by reference into this Registration Statement.

The required opinion and consents are listed on the Exhibit Index below and filed herewith.

EXHIBIT INDEX

SIGNATURES

Pursuant to the requirements of the Securities Act of 1933, as amended, the Registrant has duly caused this Registration Statement to be signed on its behalf by the undersigned, thereunto duly authorized, in Menlo Park, California on June 27, 2018.

POWER OF ATTORNEY

Pursuant to the requirements of the Securities Act, this Registration Statement has been signed by the following persons in the capacities and on the dates indicated.

Exhibit 5.1

John T. McKenna

+1 650 843 5059

jmckenna@cooley.com

June 27, 2018

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 a filing by the Company of a registration statement on Form S-1 (the “Registration Statement”) with the Securities and Exchange Commission pursuant to Rule 462(b) promulgated under the Securities Act of 1933, as amended, covering an underwritten public offering of up to 385,250 shares of the Company’s common stock, par value $0.0001 per share (the “Shares”) including 50,250 Shares that may be sold pursuant to the exercise of an option to purchase additional shares. The Registration Statement incorporates by reference the registration statement on Form S-1 (No. 333-225390), which was declared effective on June 27, 2018 (the “Prior Registration Statement”), including the prospectus which forms a part of the Prior Registration Statement (the “Prospectus”).

In connection with this opinion, we have examined and relied upon (a) the Registration Statement, the Prior Registration Statement and the Prospectus, (b) the Company’s Amended and Restated Certificate of Incorporation and Amended and Restated Bylaws, as currently in effect as of the date hereof and (c) the 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, and the conformity to originals of all documents submitted to us as copies and the due execution and delivery of all documents where due execution and delivery are a prerequisite to the effectiveness thereof (except we have not assumed the due execution and delivery by the Company of any such documents). 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.

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

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

Forty Seven, Inc.

June 27, 2018

Page Two

On the basis of the foregoing, and in reliance thereon, we are of the opinion that the Shares, when sold and issued against payment therefore in accordance with the Registration Statement and the Prospectus, will be validly issued, fully paid and non-assessable.

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

Sincerely,

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

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

Exhibit 23.1

Consent of Independent Registered Public Accounting Firm

We consent to the incorporation by reference in the Registration Statement on Form S-1 filed pursuant to Rule 462(b) of the Securities Act of 1933 of the reference to our firm under the caption “Experts” and to the incorporation by reference of our report dated March 22, 2018 (except for the last paragraph of Note 1, as to which the date is June 15, 2018), with respect to the financial statements of Forty Seven, Inc. included in the Registration Statement (Form S-1 No. 333-225390) and related Prospectus of Forty Seven, Inc. for the registration of its common stock.

/s/ Ernst & Young LLP

San Jose, California

June 25, 2018