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): February 18, 2015

Trinseo S.A.

(Exact name of registrant as specified in its charter)

001-36473

(Commission File Number)

(610) 240-3200

(Registrant’s telephone number, including area code)

Not Applicable

(Former name or former address, if changed since last report)

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:

Item 5.02 Departure of Directors or Certain Officers; Election of Directors; Appointment of Certain Officers; Compensatory Arrangements of Certain Officers.

On February 18, 2015, Stephen F. Thomas announced his intention to resign as a member of the board of directors (the “Board”) of Trinseo S.A. (the “Company”). Mr. Thomas’ resignation became effective on February 19, 2015. There were no disagreements between Mr. Thomas and the Company.

On February 19, 2015, upon the recommendation of the Company’s Nominating and Corporate Governance Committee, the Board approved the appointment of Donald T. Misheff to the Board to fill the seat vacated by Mr. Thomas, in accordance with the Company’s Articles of Association, with immediate effect. Upon ratification of his appointment by the shareholders at the 2015 Annual General Meeting of Shareholders, Mr. Misheff will hold office until the 2017 Annual General Meeting of Shareholders and until a successor is elected and qualified. Mr. Misheff was also appointed a member of the Board’s Audit Committee, replacing Brian W. Chu, who resigned his membership on the Audit Committee effective February 19, 2015. Mr. Misheff was also appointed a member of the Environmental, Health, Safety and Public Policy Committee of the Board.

There is no arrangement or understanding under which Mr. Misheff was appointed as a director. There are no transactions involving Mr. Misheff requiring disclosure under Item 404(a) of Regulation S-K. Mr. Misheff will participate in the Company’s director compensation program. In connection with his appointment to the Board, Mr. Misheff and the Company entered into the Company’s standard indemnification agreement, the terms of which are described in the Company’s Registration Statement on Form S-1 (File No. 333-194561) and a form of such agreement was filed as Exhibit 10.36 thereto.

On February 23, 2015, the Company issued a press release announcing the appointment of Mr. Misheff to the Board. A copy of the press release is furnished as Exhibit 99.1 hereto.

The information contained in this Item 7.01 and in Exhibit 99.1 shall not be deemed filed for purposes of Section 18 of the Securities Exchange Act of 1934, as amended, or otherwise subject to liability of that section, nor shall it be deemed incorporated by reference in any filing under the Securities Act of 1933, as amended, except as shall be expressly set forth by specific reference in such filing.

The 2015 Annual General Meeting of Shareholders will be held on June 25, 2015. The record date for the meeting is April 17, 2015. The deadline for receipt of shareholder proposals for inclusion in the Company’s proxy statement is March 5, 2015.

(d) Exhibits.

SIGNATURE

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

Date: February 23, 2015

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