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): May 7, 2015

KOPPERS HOLDINGS INC.

(Exact name of registrant as specified in its charter)

Registrant’s telephone number, including area code: (412) 227-2001

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 (see General Instruction A.2. below):

On May 8, 2015 we issued a press release announcing first quarter 2015 results. A copy of the press release is included in this Current Report on Form 8-K as Exhibit 99.1 and is furnished herewith.

The Koppers Holdings Inc. (“Koppers”) Annual Meeting of Shareholders (the “Annual Meeting”) was held on May 7, 2015. Four matters were considered and voted upon at the Annual Meeting: (1) the election of seven persons to serve on our board of directors; (2) an amendment to our Amended and Restated Articles of Incorporation (the “Articles”) to increase the number of authorized shares of common stock; (3) an advisory vote to approve executive compensation; and (4) the ratification of the appointment of Ernst & Young LLP as our independent registered public accounting firm for 2015.

Election of Directors: Nominations of Cynthia A. Baldwin, Leroy M. Ball, Jr., Sharon Feng, Ph.D., Albert J. Neupaver, Stephen R. Tritch, Walter W. Turner and T. Michael Young to serve as directors for a one-year term expiring in 2016 were considered and all nominees were elected. All nominees for election as director received a majority of votes cast. The final voting results are as follows:

The terms of office of David M. Hillenbrand, Ph.D. and Louis L. Testoni continued after the Annual Meeting. They will serve as directors until their terms expire in 2016 and their successors have been duly elected and qualify.

Amendment to the Articles to Increase Authorized Common Stock: The proposal to amend our Articles to increase the number of authorized shares of common stock was approved. The final voting results are as follows:

For: 14,433,006

Against: 4,734,526

Abstain: 16,486

There were no broker non-votes with respect to this matter.

Advisory Vote to Approve Executive Compensation: The advisory (non-binding) vote approving the compensation of the named executive officers of Koppers as disclosed in the Notice of Annual Meeting and Proxy Statement for the 2015 Annual Meeting was approved. The final voting results are as follows:

For: 16,822,530

Against: 629,977

Abstain: 10,922

Broker Non-Votes: 1,720,589

Ratification of Appointment of Ernst & Young LLP: The Audit Committee of the Board of Directors of Koppers appointed Ernst & Young LLP as our independent registered public accounting firm for the year 2015. The final voting results to ratify the appointment of Ernst & Young LLP are as follows:

For: 19,054,599

Against: 117,059

Abstain: 12,360

There were no broker non-votes with respect to this matter.

(d) Exhibits. The following exhibits are furnished herewith:

SIGNATURES

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.

EXHIBIT INDEX