UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, DC 20549

FORM 10-K/A

(Amendment No. 1)

(Mark One)

For the fiscal year ended September 30, 2015

OR

For the transition period from                      to                     

Commission file number 814-00237

GLADSTONE CAPITAL CORPORATION

(Exact name of registrant as specified in its charter)

(703) 287-5800

(Registrant’s telephone number, including area code)

Securities registered pursuant to Section 12(b) of the Act:

Securities registered pursuant to Section 12(g) of the Act: None

Indicate by check mark if the registrant is a well-known seasoned issuer, as defined in Rule 405 of the Securities Act.    YES  ¨    NO  x

Indicate by check mark if the registrant is not required to file reports pursuant to Section 13 or Section 15(d) of the Act.    YES  ¨    NO  x

Indicate by check mark whether the registrant (1) has filed all reports required to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during the preceding 12 months (or for such shorter period that the registrant was required to file such reports), and (2) has been subject to such filing requirements for the past 90 days.    YES  x    NO  ¨

Indicate by check mark whether the registrant has submitted electronically and posted on its corporate Web site, if any, every Interactive Data File required to be submitted and posted pursuant to Rule 405 of Regulation S-T (§232.405 of this chapter) during the preceding 12 months (or for such shorter period that the registrant was required to submit and post such files).    YES  ¨    NO  ¨

Indicate by check mark if disclosure of delinquent filers pursuant to Item 405 of Regulation S-K is not contained herein, and will not be contained, to the best of registrant’s knowledge, in definitive proxy or information statements incorporated by reference in Part III of this Form 10-K or any amendment to this Form 10-K.    ¨

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, or a smaller reporting company. See the definitions of “large accelerated filer,” “accelerated filer” and “smaller reporting company” in Rule 12b-2 of the Exchange Act. (Check one):

Indicate by check mark whether the registrant is a shell company (as defined in Rule 12 b-2 of the Act).    YES  ¨    NO  x.

The aggregate market value of the voting common stock held by non-affiliates of the Registrant on March 31, 2015, based on the closing price on that date of $8.81 on the NASDAQ Global Select Market, was $174,201,704. For the purposes of calculating this amount only, all directors and executive officers of the Registrant have been treated as affiliates. There were 23,461,622 shares of the Registrant’s common stock, $0.001 par value per share, outstanding as of November 20, 2015.

Documents Incorporated by Reference. Portions of the Registrant’s definitive proxy statement filed with the Securities and Exchange Commission pursuant to Regulation 14A in connection with the Registrant’s 2016 Annual Meeting of Stockholders are incorporated by reference into Part III of the Form 10-K. Such proxy statement was filed on December 11, 2015.

EXPLANATORY NOTE

Gladstone Capital Corporation (“we” or the “Company”) is filing this Amendment No. 1 (the “Amendment”) to our Annual Report on Form 10-K for the fiscal year ended September 30, 2015, which was filed with the Securities and Exchange Commission (the “SEC”) on November 23, 2015 (the “Form 10-K”), to provide separate unaudited financial statements for our unconsolidated portfolio company, RBC Acquisition Corp. and Subsidiary (“RBC”), as of and for the fiscal years ended September 30, 2015 and 2014 (Exhibit 99.5), separate audited financial statements for RBC as of and for the fiscal year ended September 30, 2014 (Exhibit 99.6) and separate unaudited financial statements for RBC as of and for the fiscal years ended September 2013 and 2012 (Exhibit 99.7), in Part IV, Item 15.

We have determined that this unconsolidated portfolio company has met the conditions of a significant subsidiary under Rule 1-02(w) of Regulation S-X for which we are required, pursuant to Rule 3-09 of Regulation S-X, to attach separate financial statements as exhibits to the Form 10-K. In accordance with Rule 3-09(b)(1), the separate financial statements of RBC are being filed as an amendment to the Form 10-K, within 90 days after the end of our fiscal year.

This Amendment also updates, amends and supplements Part IV, Item 15 of the Form 10-K to include, among other items, the filing of new Exhibits 31.1, 31.2, 32.1 and 32.2, certifications of our Chief Executive Officer and Chief Financial Officer, pursuant to Rule 13a-14(a) and (b).

No other changes have been made to the Form 10-K other than that described above. This Amendment does not reflect subsequent events occurring after the original filing date of the Form 10-K or modify or update in any way disclosures made in the Form 10-K. Among other things, forward-looking statements made in the Form 10-K have not been revised to reflect events that occurred or facts that became known to us after filing of the Form 10-K, and such forward-looking statements should be read in their historical context. Furthermore, this Amendment should be read in conjunction with the Form 10-K and with our filings with the SEC subsequent to the Form 10-K.

PART IV

ITEM 15. EXHIBITS AND FINANCIAL STATEMENT SCHEDULES

The following exhibits are filed as part of this report or are hereby incorporated by reference to exhibits previously filed with the SEC:

SIGNATURES

Pursuant to the requirements of Section 13 or 15(d) of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned, thereunto duly authorized.

Pursuant to the requirements of the Securities Exchange Act of 1934, this report has been signed below by the following persons on behalf of the registrant and in the capacities and on the dates indicated.