SNNA Sienna Biopharmaceuticals Inc

As filed with the Securities and Exchange Commission on March 14, 2019

Registration No. 333-                

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM S-8

REGISTRATION STATEMENT

UNDER

THE SECURITIES ACT OF 1933

2017 Incentive Award Plan

2017 Employee Stock Purchase Plan

(Full Title of the Plan)

Frederick C. Beddingfield III, M.D., Ph.D.

President and Chief Executive Officer

Sienna Biopharmaceuticals, Inc.

30699 Russell Ranch Road, Suite 140

Westlake Village, CA 91362

(818) 629-2256

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

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.

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 7(a)(2)(B) of the Securities Act.  ☒

REGISTRATION OF ADDITIONAL SECURITIES

This Registration Statement on Form S-8 is being filed for the purpose of registering additional shares of common stock, par value $0.0001 per share (the “Common Stock”) of Sienna Biopharmaceuticals, Inc. (the “Registrant”). The Registrant is hereby registering an additional 847,084 shares of Common Stock issuable under the Sienna Biopharmaceuticals, Inc. 2017 Incentive Award Plan (the “2017 Plan”) and an additional 211,771 shares of Common Stock issuable under the Sienna Biopharmaceuticals, Inc. 2017 Employee Stock Purchase Plan (the “ESPP”). The Registrant previously registered shares of its Common Stock for issuance under the 2017 Plan and the ESPP under a Registration Statement on Form S-8 filed with the Securities and Exchange Commission (the “Commission”) on August 1, 2017, (File 333-219609) and a Registration Statement on Form S-8 filed with the Commission on March 15, 2018 (File No. 333-223689) (the “Original Registration Statements”).

INCORPORATION BY REFERENCE OF CONTENTS OF

REGISTRATION STATEMENT ON FORM S-8

Pursuant to Instruction E of Form S-8, the contents of the Original Registration Statements are incorporated by reference in this registration statement on Form S-8.

INFORMATION REQUIRED IN THE SECTION 10(a) PROSPECTUS

The information called for in Part I of Form S-8 is not being filed with or included in this Form S-8 (by incorporation by reference or otherwise) in accordance with the rules and regulations of the SEC.