UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, DC 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): December 13, 2012

GOLUB CAPITAL BDC, INC.

(Exact name of Registrant as Specified in Its Charter)

Registrant’s telephone number, including area code: (312) 205-5050

(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 December 13, 2012, Golub Capital BDC Funding LLC (“Funding”), a wholly owned subsidiary of Golub Capital BDC, Inc. (the “Company”), entered into an amendment (the “Credit Facility Amendment”) to the documents governing Funding’s senior, secured revolving credit facility (the “Credit Facility”) with Wells Fargo Securities, LLC, as administrative agent, and Wells Fargo Bank, N.A., as lender.

The Credit Facility Amendment, among other things, increased the size of the Credit Facility from $75 million to $150 million.

As previously disclosed, the Credit Facility is secured by all of the assets held by Funding, and the Company has pledged its interests in Funding as collateral to Wells Fargo Bank, N.A., as the collateral agent, to secure the obligations of the Company as the transferor and servicer under the Credit Facility. Borrowing under the Credit Facility remains subject to the leverage restrictions contained in the Investment Company Act of 1940, as amended.

The description above is only a summary of the material provisions of the Credit Facility Amendment and is qualified in its entirety by reference to a copy of the Credit Facility Amendment, which is filed as Exhibit 10.1 to this current report on Form 8-K.

Item 9.01. Financial Statements and Exhibits.

(d) Exhibits.

10.1 Fifth Amendment to Loan and Servicing Agreement, dated December 13, 2012, by and among Golub Capital BDC Funding LLC, as the Borrower; Golub Capital BDC, Inc., as Transferor and Servicer; Wells Fargo Securities, LLC, as the Administrative Agent; the lenders from time to time party thereto; the lender agents from time to time party thereto; and Wells Fargo Bank, N.A., as the Collateral Agent, the Account Bank, and the Collateral Custodian (amending the Loan and Servicing Agreement, dated as of July 21, 2011).

99.1 Press Release of Golub Capital BDC, Inc. dated December 17, 2012.

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, Golub Capital BDC, Inc. has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.