UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, DC 20549





FORM 10-K/A

(Amendment No. 1)





(Mark One)

|x | ANNUAL REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934
|- | ------------------------------------------------------------------------------------

For the fiscal year ended September 30, 2015

OR



| ̈ | TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934
|- | ----------------------------------------------------------------------------------------

For the transition period from
 to

Commission file number 814-00237





GLADSTONE
CAPITAL CORPORATION

(Exact name of registrant as specified in its charter)







|                                                               |  |                                     
|-------------------------------------------------------------- |  | ------------------------------------
|Maryland                                                       |  | 54-2040781                          
|(State or other jurisdiction of
incorporation or organization) |  | (I.R.S. Employer
Identification No.)
|                                                               | 
|1521 Westbranch Drive, Suite 100
McLean, Virginia              |  | 22102                               
|(Address of principal executive offices)                       |  | (Zip Code)                          

(703) 287-5800

(Registrants telephone number, including area code)

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



|                                                                   |  |                                          
|------------------------------------------------------------------ |  | -----------------------------------------
|Title of each Class                                                |  | Name of each exchange on which registered
|Common Stock, $0.001 par value per share                           |  | NASDAQ Global Select Market              
|6.75% Series 2021 Term Preferred Stock, $0.001 par value per share |  | NASDAQ Global Select Market              

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):



|                        |  |                                                  |  |                           |  |  
|----------------------- |  | ------------------------------------------------ |  | ------------------------- |  | -
|Large accelerated filer |  |  ̈                                                |  | Accelerated filer         |  | x
|                        |  |                                                  | 
|Non-accelerated filer   |  |  ̈  (Do not check if a smaller reporting company) |  | Smaller reporting company |  |  ̈

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



|a. | DOCUMENTS FILED AS PART OF THIS REPORT
|-- | --------------------------------------



|3. | Exhibits
|-- | --------

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



|     |  |                                                                                                                                                                                                                                                                                                                                                                                                           
|---- |  | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
|3.1  |  | Articles of Amendment and Restatement to the Articles of Incorporation, incorporated by reference to Exhibit 99.a.2 to Pre-Effective Amendment No. 1 to the Registration Statement on Form N-2 (File No. 333-63700),
filed July 27, 2001.                                                                                                                                                                 
|     | 
|3.2  |  | Articles Supplementary Establishing and Fixing the Rights and Preferences of Term Preferred Shares, including Appendix A thereto relating to the Term Preferred Shares, 7.125% Series 2016, incorporated by reference to
Exhibit 2.a.2 to Post-Effective Amendment No. 5 to the Registration Statement on Form N-2 (File No. 333-162592), filed October 31, 2011.                                         
|     | 
|3.3  |  | Articles Supplementary Establishing and Fixing the Rights and Preferences of Term Preferred Shares, 6.75% Series 2021, including Appendix A thereto, incorporated by reference to Exhibit 3.3 to Form 8-A (File
No. 001-35332), filed May 15, 2014.                                                                                                                                                       
|     | 
|3.4  |  | Certificate of Correction to Articles Supplementary Establishing and Fixing the Rights and Preferences of Term Preferred Shares, 6.75% Series 2021, incorporated by reference to Exhibit 3.4 to the Quarterly Report on Form
10-Q (File No. 811-000000), filed July 30, 2014.                                                                                                                             
|     | 
|3.5  |  | Certificate of Correction to Articles Supplementary Establishing and Fixing the Rights and Preferences of Term Preferred Shares, incorporated by reference to Exhibit 3.1 to the Current Report on Form 8-K (File No. 814-00237), filed
October 29, 2015.                                                                                                                                                 
|     | 
|3.6  |  | By-laws, incorporated by reference to Exhibit 99.b to Pre-Effective Amendment No. 1 to the Registration Statement on Form N-2 (File No. 333-63700), filed July 27, 2001.                                                                                                                                                                                                                                  
|     | 
|3.7  |  | Amendment to By-laws, incorporated by reference to Exhibit 3.3 to the Quarterly Report on Form 10-Q (File No. 814-00237), filed February 17, 2004.                                                                                                                                                                                                                                                        
|     | 
|3.8  |  | Second Amendment to By-laws, incorporated by reference to Exhibit 99.1 to the Current Report on Form 8-K (File No. 814-00237), filed July 10, 2007.                                                                                                                                                                                                                                                       
|     | 
|3.9  |  | Third Amendment to By-laws, incorporated by reference to Exhibit 99.1 to the Current Report on Form 8-K (File No. 814-00237), filed June 10, 2011.                                                                                                                                                                                                                                                        
|     | 
|4.1  |  | Form of Certificate for Common Stock, incorporated by reference to Exhibit 99.d.2 to Pre-Effective Amendment No. 3 to the Registration Statement on Form N-2 (File No. 333-63700), filed August 23, 2001.                                                                                                                                                                                                 
|     | 
|4.2  |  | Form of Certificate for 6.75% Series 2021 Term Preferred Stock, incorporated by reference to Exhibit 4.3 to Form 8-A (File No. 001-35332), filed May 15, 2014.                                                                                                                                                                                                                                            
|     | 
|10.1 |  | Stock Transfer Agency Agreement between the Registrant and The Bank of New York, incorporated by reference to Exhibit 99.k.1 to Pre-Effective Amendment No. 1 to the Registration Statement on Form N-2 (File No. 333-63700), filed
July 27, 2001.                                                                                                                                                        
|     | 
|10.2 |  | Custody Agreement between the Registrant and The Bank of New York, dated as of May 5, 2006, incorporated by reference to Exhibit 10.3 to the Quarterly Report on Form 10-Q (File No. 814-00237), filed August 1,
2006.                                                                                                                                                                                    
|     | 
|10.3 |  | Amended and Restated Investment Advisory and Management Agreement between the Registrant and Gladstone Management Corporation, dated as of October 1, 2006, incorporated by reference to Exhibit 99.1 to the Current Report on Form
8-K (File No. 814-00237), filed October 5, 2006.                                                                                                                      
|     | 
|10.4 |  | Amendment No. 1 to Amended and Restarted Investment Advisory and Management Agreement between the Registrant and Gladstone Management Corporation, dated as of October 13, 2015 incorporated by reference to Exhibit 10.1 to the
Current Report on Form 8-K (File No. 814-00237), filed October 14, 2015.                                                                                                 
|     | 
|10.5 |  | Administration Agreement between the Registrant and Gladstone Administration, LLC, dated as of October 1, 2006, incorporated by reference to Exhibit 99.2 to the Current Report on Form 8-K (File No. 814-00237), filed
October 5, 2006.                                                                                                                                                                  
|     | 
|10.6 |  | Fifth Amended and Restated Credit Agreement dated as of May 1, 2015, by and among Gladstone Business Loan, LLC, as Borrower, Gladstone Management Corporation, as Servicer, the Lenders and Managing Agents named therein, and
Keybank National Association, as Administrative Agent, incorporated by reference to Exhibit 10.1 to the Current Report on Form 8-K (File No. 814-00237), filed May 5, 2015.
|     | 
|10.7 |  | Joinder Agreement, dated as of June 19, 2015, by and among Gladstone Business Loan, LLC, Gladstone Management Corporation, Keybank National Association and Santander Bank, N.A., incorporated by reference to Exhibit 10.1 to the
Current Report on Form 8-K (File No. 814-00237), filed June 23, 2015.                                                                                                  
|     | 
|10.8 |  | Assignment, Acceptance and Joinder, dated as of June 19, 2015, by and between Keybank National Association and Alostar Bank of Commerce, incorporated by reference to Exhibit 10.2 to the Current Report on Form 8-K (File No.
814-00237), filed June 23, 2015.                                                                                                                                           



---

|      |  |                                                                                                                                                                                                                                             
|----- |  | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
|10.9  |  | Assignment and Acceptance, dated as of June 19, 2015, by and between Keybank National Association and Newbridge Bank, incorporated by reference to Exhibit 10.3 to the Current Report on Form 8-K (File No. 814-00237), filed
June 23, 2015.
|      | 
|10.10 |  | Custodial Agreement, incorporated by reference to Exhibit 2.j.2 to Post-Effective Amendment No. 1 to Form N-2 (File No. 333-185191), filed December 23, 2013.                                                                               
|      | 
|10.11 |  | Amendment No. 1 to Custodial Agreement, incorporated by reference to Exhibit 2.j.3 to Post-Effective Amendment No. 1 to the Registration Statement on Form N-2 (File No. 333-185191), filed December 23, 2013.                              
|      | 
|10.12 |  | Amendment No. 2 to Custodial Agreement, incorporated by reference to Exhibit 2.j.4 to Post-Effective Amendment No. 1 to the Registration Statement on Form N-2 (File No. 333-185191), filed December 23, 2013.                              
|      | 
|11*   |  | Computation of Per Share Earnings.                                                                                                                                                                                                          
|      | 
|12*   |  | Statements Re: Computation of Ratios.                                                                                                                                                                                                       
|      | 
|14    |  | Code of Ethics and Business Conduct, updated January 28, 2013, incorporated by reference to Exhibit 14 to the Annual Report on Form 10-K (File No. 814-00237), filed November 20, 2013.                                                     
|      | 
|21*   |  | Subsidiaries of the Registrant.                                                                                                                                                                                                             
|      | 
|31.1  |  | Certification of Chief Executive Officer pursuant to section 302 of The Sarbanes-Oxley Act of 2002 (filed herewith).                                                                                                                        
|      | 
|31.2  |  | Certification of Chief Financial Officer pursuant to section 302 of The Sarbanes-Oxley Act of 2002 (filed herewith).                                                                                                                        
|      | 
|32.1  |  | Certification of Chief Executive Officer pursuant to section 906 of The Sarbanes-Oxley Act of 2002 (furnished herewith).                                                                                                                    
|      | 
|32.2  |  | Certification of Chief Financial Officer pursuant to section 906 of The Sarbanes-Oxley Act of 2002 (furnished herewith).                                                                                                                    
|      | 
|99.1* |  | Financial Statements of Defiance Integrated Technologies, Inc. as of and for the years ended December 31, 2014 and 2013 (unaudited).                                                                                                        
|      | 
|99.2* |  | Financial Statements of Defiance Integrated Technologies, Inc. as of and for the years ended December 31, 2013 and 2012 (audited).                                                                                                          
|      | 
|99.3* |  | Financial Statements of Midwest Metal Distribution, Inc. and Subsidiaries as of and for the years ended December 31, 2013 and 2012 (audited).                                                                                               
|      | 
|99.4* |  | Financial Statements of Sunshine Media Group, Inc. as of and for the years ended December 31, 2014 and 2013 (audited).                                                                                                                      
|      | 
|99.5  |  | Financial Statements of RBC Acquisition Corp. and Subsidiary as of and for the years ended September 30, 2015 and 2014 (unaudited).                                                                                                         
|      | 
|99.6  |  | Financial Statements of RBC Acquisition Corp. and Subsidiary as of and for the year ended September 30, 2014 (audited).                                                                                                                     
|      | 
|99.7  |  | Financial Statements of RBC Acquisition Corp. and Subsidiary as of and for the years ended September 30, 2013 and 2012 (unaudited).                                                                                                         



|* | Filed November 23, 2015, with the Companys Annual Report on Form 10-K (File No. 814-00237), for the year ended September 30, 2015, and incorporated by reference herein.
|- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------



---

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.



|                        |  |  |  |                               |  |                        
|----------------------- |  |  |  | ----------------------------- |  | -----------------------
|                        |  |  |  | GLADSTONE CAPITAL CORPORATION
|                        |  |  | 
|Date: December 29, 2015 |  |  |  | By:                           |  | /s/ MELISSA MORRISON   
|                        |  |  |  |                               |  | Melissa Morrison       
|                        |  |  |  |                               |  | Chief Financial Officer

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.



|                        |  |  |  |     |  |                                                                                             
|----------------------- |  |  |  | --- |  | --------------------------------------------------------------------------------------------
|Date: December 29, 2015 |  |  |  | By: |  | /s/ DAVID GLADSTONE                                                                         
|                        |  |  |  |     |  | David Gladstone                                                                             
|                        |  |  |  |     |  | Chief Executive Officer and Chairman of the Board of Directors (principal executive officer)
|                        |  |  | 
|Date: December 29, 2015 |  |  |  | By: |  | /s/ TERRY L. BRUBAKER                                                                       
|                        |  |  |  |     |  | Terry L. Brubaker                                                                           
|                        |  |  |  |     |  | Vice Chairman of the Board of Directors, Chief Operating Officer                            
|                        |  |  | 
|Date: December 29, 2015 |  |  |  | By: |  | /s/ ROBERT L. MARCOTTE                                                                      
|                        |  |  |  |     |  | Robert L. Marcotte                                                                          
|                        |  |  |  |     |  | President                                                                                   
|                        |  |  | 
|Date: December 29, 2015 |  |  |  | By: |  | /s/ MELISSA MORRISON                                                                        
|                        |  |  |  |     |  | Melissa Morrison                                                                            
|                        |  |  |  |     |  | Chief Financial Officer (principal financial and accounting officer)                        
|                        |  |  | 
|Date: December 29, 2015 |  |  |  | By: |  | /s/ ANTHONY W. PARKER                                                                       
|                        |  |  |  |     |  | Anthony W. Parker                                                                           
|                        |  |  |  |     |  | Director                                                                                    
|                        |  |  | 
|Date: December 29, 2015 |  |  |  | By: |  | /s/ JOHN OUTLAND                                                                            
|                        |  |  |  |     |  | John Outland                                                                                
|                        |  |  |  |     |  | Director                                                                                    
|                        |  |  | 
|Date: December 29, 2015 |  |  |  | By: |  | /s/ MICHELA A. ENGLISH                                                                      
|                        |  |  |  |     |  | Michela A. English                                                                          
|                        |  |  |  |     |  | Director                                                                                    
|                        |  |  | 
|Date: December 29, 2015 |  |  |  | By: |  | /s/ PAUL ADELGREN                                                                           
|                        |  |  |  |     |  | Paul Adelgren                                                                               
|                        |  |  |  |     |  | Director                                                                                    
|                        |  |  | 
|Date: December 29, 2015 |  |  |  | By: |  | /s/ WALTER H. WILKINSON, JR.                                                                
|                        |  |  |  |     |  | Walter H. Wilkinson, Jr.                                                                    
|                        |  |  |  |     |  | Director                                                                                    
|                        |  |  | 
|Date: December 29, 2015 |  |  |  | By: |  | /s/ CAREN D. MERRICK                                                                        
|                        |  |  |  |     |  | Caren D. Merrick                                                                            
|                        |  |  |  |     |  | Director