As filed with the Securities and Exchange Commission on January 28, 2015

Registration No. 333-          

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM S-1

REGISTRATION STATEMENT

UNDER

THE SECURITIES ACT OF 1933

Flex Pharma, Inc.

(Exact Name of Registrant as Specified in Its Charter)

800 Boylston Street, 24th Floor

Boston, MA 02199

(617) 874-1821

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

Christoph Westphal, M.D., Ph.D.

President and Chief Executive Officer

Flex Pharma, Inc.

800 Boylston Street, 24th Floor

Boston, MA 02199

(617) 874-1821

(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 this registration statement becomes effective.

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, as amended (the “Securities Act”), check the following box.  o

If this Form is filed to register additional securities for an offering pursuant to Rule 462(b) 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.  x (File No. 333-201276)

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.  o

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.  o

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

CALCULATION OF REGISTRATION FEE

(1)          The shares being registered pursuant to this Registration Statement are in addition to the 5,307,250 shares registered pursuant to the Registrant’s Registration Statement on Form S-1 (Registration No. 333-201276). Includes 117,750 shares that the underwriters have the option to purchase.

(2)   Based on the initial public offering price.

(3)   Calculated pursuant to Rule 457(a) under the Securities Act of 1933, as amended.

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

EXPLANATORY NOTE AND

INCORPORATION OF CERTAIN INFORMATION BY REFERENCE

This Registration Statement is being filed with the Securities and Exchange Commission (the “Commission”) with respect to the registration of additional shares of common stock, par value $0.0001 per share, of Flex Pharma, Inc., a Delaware corporation,  pursuant to Rule 462(b) under the Securities Act of 1933, as amended. This Registration Statement incorporates by reference the contents of, including all amendments and exhibits thereto and all information incorporated by reference therein, the Registration Statement on Form S-1 (Registration No. 333-201276), which was declared effective by the Commission on January 28, 2015, and is being filed solely for the purpose of increasing the aggregate number of shares to be offered in the public offering by 902,750 shares including the shares that may be sold pursuant to the underwriters’ option to purchase additional shares.

The required opinions and consents are listed on the Exhibit Index attached hereto.

SIGNATURES

Pursuant to the requirements of the Securities Act, the Registrant has duly caused this registration statement to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of Boston, State of Massachusetts, on the 28th day of January, 2015.

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.

*Pursuant to power of attorney

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EXHIBIT INDEX

(1)                                 Included on the signature page of Registration Statement on Form S-1 (File No. 333-201276), filed with the Securities and Exchange Commission on January 13, 2015, and incorporated herein by reference.

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