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 4, 2015

Spark Therapeutics, Inc.

(Exact Name of Registrant as Specified in its Charter)

Registrant’s telephone number, including area code: (888) 772-7560

(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):

Item 5.03. Amendments to Articles of Incorporation or Bylaws; Change in Fiscal Year.

On February 4, 2015, Spark Therapeutics, Inc. (the “Company”) issued and sold shares of its common stock in connection with the closing of the Company’s initial public offering (the “IPO”). On the same day, the Company filed a restated certificate of incorporation (the “Restated Certificate”) with the Secretary of State of the State of Delaware in connection with closing of the IPO. The Company’s board of directors and stockholders previously approved the Restated Certificate to be filed in connection with, and to be effective upon, the closing of the IPO.

The Restated Certificate amends and restates the Company’s certificate of incorporation in its entirety to, among other things: (i) increase the authorized number of shares of common stock to 150,000,000 shares; (ii) eliminate all references to the previously existing series of preferred stock; (iii) authorize 5,000,000 shares of undesignated preferred stock that may be issued from time to time by the Company’s board of directors in one or more series; (iv) establish a classified board of directors, divided into three classes, each of whose members will serve for staggered three-year terms; (v) provide that directors may be removed from office only for cause and only upon the affirmative vote of the holders of at least 75% of the votes that all of the Company’s stockholders would be entitled to cast in an annual election of directors; (vi) provide that any vacancy on the Company’s board of directors, including a vacancy resulting from an enlargement of its board of directors, may be filled only by vote of a majority of directors then in office; and (vii) eliminate the ability of the Company’s stockholders to take action by written consent in lieu of a meeting and call special meetings of stockholders.

The foregoing description of the amendments made by the Restated Certificate is qualified by reference to the Restated Certificate, a copy of which is attached hereto as Exhibit 3.1 and is incorporated herein by reference.

On February 4, 2015, in connection with the closing of the IPO, amended and restated by-laws of the Company (the “Amended and Restated By-Laws”), previously approved by the Company’s board of directors and stockholders to become effective immediately following the closing of the IPO, became effective. The Amended and Restated By-Laws amend and restate the Company’s by-laws in their entirety to, among other things: (i) eliminate the ability of the Company’s stockholders to take action by written consent in lieu of a meeting and call special meetings of stockholders; (ii) establish procedures relating to the presentation of stockholder proposals at stockholder meetings; (iii) establish procedures relating to the nomination of directors; and (iv) conform to the amended provisions of the Restated Certificate.

The foregoing description of the amendments made in the Amended and Restated By-Laws is qualified by reference to the Amended and Restated By-Laws, a copy of which is attached hereto as Exhibit 3.2 and is incorporated herein by reference.

Item 9.01. Financial Statements and Exhibits.

(d) Exhibits

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