UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d)

of the Securities Exchange Act of 1934

Date of Report (Date of earliest event reported): June 1, 2018

SIENNA BIOPHARMACEUTICALS, INC.

(Exact name of registrant as specified in its charter)

30699 Russell Ranch Road, Suite 140

Westlake Village, CA 91362

(Address of principal executive offices, including Zip Code)

Registrant’s telephone number, including area code: (818) 629-2256

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions:

Indicate by check mark whether the registrant is an emerging growth company as defined in Rule 405 of the Securities Act of 1933 (§ 230.405 of this chapter) or Rule 12b-2 of the Securities Exchange Act of 1934 (§ 240.12b-2 of this chapter).

Emerging growth company  ☒

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 13(a) of the Exchange Act.  ☒

On June 1, 2018, Sienna Biopharmaceuticals, Inc. (the “Company”) held its Annual Meeting of Stockholders (the “Annual Meeting”). At the Annual Meeting, the Company’s stockholders voted on two proposals, each of which is described in more detail in the Company’s definitive proxy statement on Schedule 14A filed with the Securities and Exchange Commission on April 19, 2018. Only stockholders of record as of the close of business on April 10, 2018, the record date for the Annual Meeting, were entitled to vote at the Annual Meeting. As of the record date, 20,732,399 shares of the Company’s common stock were outstanding and entitled to vote at the Annual Meeting. The tabulation of the stockholder votes on each proposal brought before the Annual Meeting is as follows:

Proposal 1. The election of two Class I directors to hold office until the 2021 annual meeting of stockholders or until their respective successor is elected and qualified:

Proposal 2. The ratification of the selection, by the audit committee of the Company’s board of directors, of Ernst & Young LLP, as the Company’s independent registered public accounting firm for its fiscal year ending December 31, 2018:

As a routine proposal under applicable rules, no broker non-votes were recorded in connection with this proposal.

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, as amended, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.