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): September 19, 2013 (September 13, 2013)

APOLLO INVESTMENT CORPORATION

(Exact name of Registrant as specified in its charter)

9 West 57^th Street, New York, NY 10019

(Address, including zip code, of principal executive offices)

Registrant’s telephone number, including area code: (212) 515-3450

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 September 13, 2013, Apollo Investment Corporation (the “Company”) amended and restated its senior secured, multi-currency, revolving credit facility (the “Amended & Restated Facility”). The Amended & Restated Facility increases the lenders’ commitments to $1,250,000,000 and extends the commitments through August 2017, and allows the Company to seek additional commitments from new and existing lenders in the future, up to an aggregate facility size not to exceed $1,710,000,000. The final maturity date of the Amended & Restated Facility is August 31, 2018. Commencing September 30, 2017, the Company is required to repay, in twelve consecutive monthly installments of equal size, the outstanding amount under the Amended & Restated Facility as of August 31, 2017. Pricing for Alternate Base Rate (ABR) borrowings will be 100 basis points over the greater of (a) the applicable Prime Rate, (b) the Federal Funds effective Rate plus 1/2 of 1% and (c) the one-month LIBO Rate plus 1%, and pricing for eurocurrency borrowings will be 200 basis points over the LIBO Rate. Terms used in the foregoing sentence have the meanings set forth in the Amended & Restated Facility.

The description above is only a summary of the material provisions of the Amended & Restated Facility and does not purport to be complete and is qualified in its entirety by reference to the provisions in such Amended & Restated Facility, which is attached hereto as Exhibit 10.1.

The disclosure set forth above under 1.01 is incorporated by reference herein.

The Registrant issued a press release, filed herewith as Exhibit 99.1, and by this reference incorporated herein, on September 16, 2013 announcing the Amended & Restated Facility.

The information disclosed under this Item 7.01, including Exhibit 99.1 hereto, is being furnished and shall not be deemed to be “filed” for purposes of Section 18 of the Securities Exchange Act of 1934 and shall not be deemed incorporated by reference into any filing made under the Securities Act of 1933, except as expressly set forth by specific reference in such filing.

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(d) Exhibits

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SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, as amended, the Registrant has duly caused this Current Report on Form 8-K to be signed on its behalf by the undersigned hereunto duly authorized.

INDEX TO EXHIBITS