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): July 10, 2015

GFI GROUP INC.

(Exact name of Registrant as specified in its charter)

55 Water Street, New York, NY 10041

(Address of principal executive offices)

Registrant’s telephone number, including area code (212) 968-4100

(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):

Effective July 10, 2015, GFI Group Inc. (“GFI”) and BGC Partners, Inc. (“BGC Partners”) entered into a guarantee (the “Guarantee”) pursuant to which BGC Partners has guaranteed the obligations of GFI under GFI’s 8.375% Senior Notes due 2018 in the remaining aggregate principal amount of $240,000,000 (the “Notes”) and the indenture, dated as of July 19, 2011 (the “Indenture”), between GFI and The Bank of New York Mellon Trust Company, N.A., as Trustee. The Guarantee is expected to have a positive impact on the credit ratings of the Notes. As previously disclosed, pursuant to the terms of the Indenture, the current interest rate on the Notes will be reduced effective July 20, 2015 as a result of prior ratings increases following the acquisition of GFI by BGC Partners. Additional further ratings increases due to the Guarantee would further reduce the interest rate of the Notes.

BGC Partners owns approximately 67% of GFI and currently consolidates the financial statements of GFI. BGC Partners and GFI will share any cost savings, including interest and costs, resulting from the credit enhancement provided by BGC Partners.

The foregoing description of the Guarantee does not purport to be complete and is qualified in its entirety by reference to the full text of such document attached as Exhibit 4.1 to this Current Report and incorporated by reference herein.

Discussion of Forward-Looking Statements by GFI and BGC Partners

Statements in this Current Report regarding GFI and BGC Partners that are not historical facts are “forward looking statements” that involve risks and uncertainties. Except as required by law, neither GFI nor BGC Partners undertakes any obligation to release any revisions to any forward-looking statements. For a discussion of additional risks and uncertainties, which could cause actual results or events to differ from those contained in the forward-looking statements, see GFI’s and BGC Partners’ Securities and Exchange Commission filings, including, but not limited to, the risk factors set forth in its public filings, including its most recent Form 10-K and any updates to such risk factors contained in subsequent Form 10-Q or Form 8-K filings.

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.

[Signature Page to Form 8-K, dated July 10, 2015, regarding BGC Partners’

Guarantee of GFI 8.375% Notes]

Exhibit List