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): May 5, 2015

Gladstone Investment Corporation

(Exact name of registrant as specified in its charter)

Registrant’s telephone number, including area code: (703) 287-5800

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:

Set forth below are certain preliminary estimates of the financial condition and results of operations of Gladstone Investment Corporation (the “Company”) for the year ended March 31, 2015. These estimates are subject to the completion of the Company’s financial closing procedures and are not a comprehensive statement of the financial results for the year ended March 31, 2015. The Company advises you that its actual results may differ materially from these estimates as a result of the completion of its financial closing procedures, final adjustments and other developments arising between now and the time that the financial results for the year ended March 31, 2015 are finalized.

Net investment income per weighted average share of common stock outstanding is estimated to have totaled between $0.72 and $0.77 for the year ended March 31, 2015.

Net asset value per share of common stock outstanding as of March 31, 2015 is estimated to be between $9.11 and $9.21.

The Company intends to announce final results of operations for the three months and year ended March 31, 2015 on May 20, 2015 after the close of the financial markets.

The preliminary financial data included herein has been prepared by, and is the responsibility of, the Company’s management. PricewaterhouseCoopers LLP, the Company’s independent registered public accounting firm, has not audited, reviewed, compiled or performed any procedures with respect to the accompanying preliminary financial data. Accordingly, PricewaterhouseCoopers LLP does not express an opinion or any other form of assurance with respect thereto.

The information in this Current Report on Form 8-K is being furnished and shall not be deemed “filed” for purposes of Section 18 of the Securities Exchange Act of 1934, as amended (the “Exchange Act”), or incorporated by reference in any filing under the Securities Act of 1933, as amended, or the Exchange Act, regardless of any general incorporation language in such filing.

Forward-Looking Statements

This communication contains forward-looking statements which are based upon the current expectations of the Company and its management and are inherently uncertain. Actual performance and results could vary materially from these estimates and projections of the future. Such statements speak only as of the time when made and are based on information available to the Company as of the date hereof and are qualified in their entirety by this cautionary statement. The Company undertakes no obligation to update the information contained herein to reflect subsequently occurring events or circumstances, except as required by applicable securities laws and regulations. For a description of certain risks to which the Company is or may be subject, please refer to the factors discussed under the captions “Forward-Looking Statements” and “Risk Factors” included in the Company’s filings with the SEC (accessible at www.sec.gov).

SIGNATURE

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.