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 10, 2014

GSI GROUP INC.

(Exact name of registrant as specified in its charter)

Registrant’s telephone number, including area code (781) 266-5700

N/A

(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 1.01. Entry into a Material Definitive Agreement

On February 10, 2014, GSI Group Inc. (the “Company”) and the Company’s wholly owned subsidiaries, GSI Group Corporation (“GSI”) and NDS Surgical Imaging, LLC (“NDS”), as the Borrowers, each of the Company’s other subsidiaries party thereto as guarantors (the “Guarantors”), each lender party thereto, and Bank of America, N.A., as Administrative Agent, entered into an amendment (the “Fourth Amendment”) to the Amended and Restated Credit Agreement, dated as of December 27, 2012, by and among the Company, GSI, and NDS, Bank of America, N.A., as Administrative Agent, Swing Line Lender, L/C Issuer and lender, Silicon Valley Bank, as Syndication Agent and lender, HSBC Bank USA, N.A., as Documentation Agent and lender, and JPMorgan Chase Bank, N.A., TD Bank, N.A., and Brown Brothers Harriman & Co., each as lenders (as amended prior to the Fourth Amendment, the “Amended and Restated Credit Agreement”).

The Fourth Amendment increases the revolving credit facility commitment under the Amended and Restated Credit Agreement by $100 million from $75 million to $175 million and resets the accordion feature to $100 million for future expansion. Additionally, the Fourth Amendment increases the maximum permitted consolidated leverage ratio from 2.75 to 3.00, and increases the maximum consolidated leverage ratio for permitted acquisitions and stock repurchases from 2.25 to 2.50. Certain other technical changes were made to the Amended and Restated Credit Agreement as a result of the Fourth Amendment and are not considered material.

The foregoing description of the Fourth Amendment does not purport to be complete and is qualified in its entirety by the full agreement attached as Exhibit 10.1 hereto and incorporated herein by reference.

The information set forth in Item 1.01 in connection with the Fourth Amendment is incorporated in this Item 2.03 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.