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) October 18, 2013

Tower Group International, Ltd.

(Exact name of registrant as specified in its charter)

Bermuda Commercial Bank Building

19 Par-La-Ville Road

Hamilton, HM 11, Bermuda

(Address of principal executive office)

(441) 279-6610

(Registrant’s telephone number, including area code)

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 October 22, 2013, four wholly-owned subsidiaries of Tower Group International, Ltd. (the “Company”), including Tower Insurance Company of New York, CastlePoint Insurance Company, CastlePoint National Insurance Company and Hermitage Insurance Company (each a “Borrower”), entered into an Amendment Agreement (the “Amendment”) with Barclays Bank PLC (“Barclays”) and Bank of Montreal, London Branch, as issuing banks (the “Issuing Banks”), and Barclays as agent and security agent, to the Letter of Credit Facility Agreement, dated as of November 11, 2011, as amended on March 12, 2012, as amended on July 16, 2012, as amended on November 21, 2012, as amended and restated on March 7, 2013, and as amended on May 2, 2013 (as so amended, the “Credit Agreement”). Pursuant to the Amendment, the parties thereto also agreed, among other matters, to amend the definition of eligible collateral.

Each Borrower and the Issuing Banks also entered into a waiver letter, dated as of October 18, 2013 (the “Waiver Letter”), relating to the Credit Agreement. Pursuant to the Waiver Letter, the Issuing Banks have agreed to waive the Borrowers’ non-compliance with certain covenants contained in the Credit Agreement regarding the Borrowers’ delivery to the Issuing Banks of certain financial statements and related compliance certificates. The foregoing summary of the Amendment, the Waiver Letter and the documents relating thereto is not complete and is qualified in its entirety by the copies filed as exhibits to this Form 8-K, which exhibits are incorporated by reference to this Item 1.01.

See Item 1.01 above.

- 2 -

- 3 -

SIGNATURES

Pursuant to the requirement 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.

- 4 -