Table of Contents

Filed pursuant to Rule 497
File No. 333-208637

PROSPECTUS SUPPLEMENT

(to Prospectus dated March 29, 2016)

2,000,000 Shares

Common Stock

We are offering 2,000,000 shares of our common stock. We are an externally managed, closed-end, non-diversified management investment company that has elected to be regulated as a business development company under the Investment Company Act of 1940, as amended. Our common stock is traded on the NASDAQ Global Select Market (“NASDAQ”) under the symbol “GLAD.” Our investment objectives are to: (1) achieve and grow current income by investing in debt securities of established businesses that we believe will provide stable earnings and cash flow to pay expenses, make principal and interest payments on our outstanding indebtedness and make distributions to stockholders that grow over time; and (2) provide our stockholders with long-term capital appreciation in the value of our assets by investing in equity securities of established businesses that we believe can grow over time to permit us to sell our equity investments for capital gains.

These shares will be offered at a discount from our most recently estimated net asset value (“NAV”) per share pursuant to authority granted for twelve months by our common stockholders at our annual meeting of stockholders held on February 11, 2016, and as subsequently approved by our Board of Directors. Our current authority to offer shares at a price below NAV per share ends on the one year anniversary of our 2016 annual meeting of stockholders, unless our stockholders vote to extend this authority at our 2017 annual meeting of stockholders. Our stockholders did not specify a maximum discount below NAV at which we are able to issue our common stock, although the number of shares sold in each offering may not exceed 25% of our outstanding common stock immediately prior to such sale. The last reported closing price of our common stock on October 25, 2016 was $8.19 per share. The estimated NAV per share of our common stock at October 24, 2016 was $8.10, which represents the midpoint in our estimated NAV per share range of $8.05 and $8.15. Sales of common stock at prices below NAV per share dilute the interest of existing stockholders, having the effect of reducing our NAV per share and may reduce our market price per share. See “Risk Factors” beginning on page S-12 of this prospectus supplement and on page 13 of the accompanying prospectus and “Sales of Common Stock Below Net Asset Value” beginning on page S-18 of this prospectus supplement and page 62 of the accompanying prospectus.

The securities in which we invest generally would be rated below investment grade if they were rated by rating agencies. Below investment grade securities, which are often referred to as “junk,” have predominantly speculative characteristics with respect to the issuer’s capacity to pay interest and repay principal. They may also be difficult to value and are illiquid.

Investing in our common stock involves a high degree of risk including, among other things, risks relating to investments in securities of small, private and developing businesses. You could lose some or all of your investment. You should carefully consider each of the factors described under “Risk Factors” beginning on page S-12 of this prospectus supplement and beginning on page 13 of the accompanying prospectus before you invest in our common stock.

This prospectus supplement and the accompanying prospectus contain important information you should know before investing in our common stock, including information about risks. Please read it before you invest and retain it for future reference. Additional information about us, including our annual, quarterly and current reports, has been filed with the Securities and Exchange Commission, or the SEC, and can be accessed at its website at www.sec.gov. This information is also available free of charge by calling us collect at (703) 287-5893 or on our corporate website located at www.gladstonecapital.com. You may also call us collect at this number to request other information. See “Additional Information” in the accompanying prospectus. The SEC has not approved or disapproved these securities or passed upon the adequacy of this prospectus supplement or the accompanying prospectus. Any representation to the contrary is a criminal offense.

The underwriters are expected to deliver the shares on or about October 31, 2016.

Joint Book-Running Managers

Co-Managers

Prospectus Supplement dated October 26, 2016

Table of Contents

ABOUT THIS PROSPECTUS SUPPLEMENT

This prospectus supplement, together with the accompanying prospectus, sets forth the information that you should know before investing in our common stock.

We also file annual reports on Form 10-K, quarterly reports on Form 10-Q and current reports on Form 8-K, proxy statements and other information with the SEC under the Securities Exchange Act of 1934, as amended, or the Exchange Act. You may inspect such reports, proxy statements and other information, as well as this prospectus supplement, and the accompanying prospectus and the exhibits and schedules to the registration statement of which the accompanying prospectus is a part, at the public reference facilities maintained by the SEC at 100 F Street, N.E., Washington, D.C. 20549. You may obtain information about the operation of the public reference facilities by calling the SEC at 1-800-SEC-0330. The SEC maintains a website that contains reports, proxy statements and other information regarding registrants, including us, that file such information electronically with the SEC. The address of the SEC’s website is www.sec.gov. You may also obtain copies of such material from the Public Reference Section of the SEC at 100 F Street, N.E., Washington, D.C. 20549, at prescribed rates.

You may request a free copy of this prospectus supplement, the accompanying prospectus, our annual reports to stockholders, when available, and other information about us, and make stockholder inquiries by calling (866) 366-5745 or by writing to us at 1521 Westbranch Drive, Suite 100, McLean, Virginia 22102, or from our website (www.GladstoneCapital.com). The information contained in, or that can be accessed through, our website is not part of this prospectus supplement or the accompanying prospectus. We make available free of charge on our website our annual report on Form 10-K, quarterly reports on Form 10-Q, current reports on Form 8-K and all amendments to those reports as soon as reasonably practicable after such material is electronically filed with or furnished to the SEC. We also furnish to our stockholders annual reports, which include annual financial information that has been examined and reported on, with an opinion expressed, by our independent registered public accounting firm.

This prospectus supplement, which describes the specific terms of this offering, also adds to and updates information contained in the accompanying prospectus. The accompanying prospectus gives more general information, some of which may not apply to this offering. If the description of this offering varies between this prospectus supplement and the accompanying prospectus, you should rely on the information contained in this prospectus supplement. However, if any statement in one of these documents is inconsistent with a statement in another document having a later date, the statement in the document having the later date modifies or supersedes the earlier statement.

You should rely only on the information contained in this prospectus supplement and the accompanying prospectus in making an investment decision. Neither we, nor the underwriters, have authorized any other person to provide you with different or inconsistent information. If anyone provides you with different or inconsistent information, you should not rely on it. We are not, and the underwriters are not, making an offer to sell shares of our common stock in any jurisdiction where such an offer or sale is not permitted. The information appearing in this prospectus supplement and in the accompanying prospectus is accurate only as of the dates on their respective covers, regardless of the time of delivery or any sale of the common stock.

Table of Contents

TABLE OF CONTENTS

Table of Contents

S-1

Table of Contents

S-2

Table of Contents

S-3

Table of Contents

S-4

Table of Contents

S-5

Table of Contents

S-6

Table of Contents

S-7

Table of Contents

FEES AND EXPENSES

The following table is intended to assist you in understanding the costs and expenses that an investor in this offering will bear directly or indirectly. We caution you that some of the percentages indicated in the table below are estimates and may vary. Except where the context suggests otherwise, whenever this prospectus supplement contains a reference to fees or expenses paid by “us” or “Gladstone Capital,” or that “we” will pay fees or expenses, stockholders will indirectly bear such fees or expenses as investors in Gladstone Capital. The following percentages are annualized and have been calculated based on actual expenses incurred in the quarter ended June 30, 2016, and average net assets attributable to common stockholders for the quarter ended June 30, 2016.

Under the Advisory Agreement, the Adviser has provided and continues to provide managerial assistance to our portfolio companies. It may also provide services other than managerial assistance to our portfolio companies and receive fees therefor. Such services may include, but are not limited to: (i) assistance obtaining, sourcing or structuring credit facilities, long term loans or additional equity from unaffiliated

S-8

Table of Contents

third parties; (ii) negotiating important contractual financial relationships; (iii) consulting services regarding restructuring of the portfolio company and financial modeling as it relates to raising additional debt and equity capital from unaffiliated third parties; and (iv) primary role in interviewing, vetting and negotiating employment contracts with candidates in connection with adding and retaining key portfolio company management team members. Generally, at the end of each quarter, 100.0% of these fees are voluntarily, irrevocably and unconditionally credited against the base management fee that we would otherwise be required to pay to the Adviser; however, a small percentage of certain of such fees, primarily for valuation of the portfolio company, is retained by the Adviser in the form of reimbursement at cost for certain tasks completed by personnel of the Adviser. For the quarter ended June 30, 2016, the base management fee credit was $0.3 million. See “Management—Certain Transactions” in the accompanying prospectus.

From time to time, the Adviser has voluntarily, irrevocably and unconditionally agreed to waive a portion of the incentive fees, to the extent net investment income did not cover 100.0% of the distributions to common stockholders during the period. For the quarter ended June 30, 2016, the incentive fee credit was $0.2 million. There can be no guarantee that the Adviser will continue to credit any portion of the fees under the Advisory Agreement in the future.

Examples of how the incentive fee would be calculated are as follows:

S-9

Table of Contents

= 100% × (2.00% - 1.75%)

= 0.25%

= (100% × (“catch-up”: 2.1875% - 1.75%)) + (20% × (2.30% - 2.1875%))

= (100% × 0.4375%) + (20% × 0.1125%)

= 0.4375% + 0.0225%

= 0.46%

= 20% × (6% - 1%)

= 20% × 5%

= 1%

For a more detailed discussion of the calculation of the two-part incentive fee, see “Management—Certain Transactions—Investment Advisory and Management Agreement” in the accompanying prospectus.

S-10

Table of Contents

Example

The following examples demonstrate the projected dollar amount of total cumulative expenses that would be incurred over various periods with respect to a hypothetical investment in our common stock. In calculating the following expense amounts, we have assumed that our gross annual operating expenses would remain at the levels set forth in the table above and are gross of any credits to any fees. The examples below and the expenses in the table above should not be considered a representation of our future expenses, and actual expenses (including the cost of debt, incentive fees, if any, and other expenses) may be greater or less than those shown. While the example assumes, as required by the SEC, a 5.00% annual return, our performance will vary and may result in a return greater or less than 5.00%.

S-11

Table of Contents

RISK FACTORS

There are material limitations with making preliminary estimates of our financial results for the three months and year ended September 30, 2016 prior to the completion of our and our auditors’ financial review procedures for such period.

The preliminary financial estimates contained in “Prospectus Supplement Summary—Recent Developments” are not a comprehensive statement of our financial results for the three months and year ended September 30, 2016 and have not been audited by our independent registered public accounting firm. Our consolidated financial statements for the year ended September 30, 2016 will not be available until after this offering is completed and, consequently, will not be available to you prior to investing in this offering. Our actual financial results for the three months and year ended September 30, 2016 may differ materially from the preliminary financial estimates we have provided as a result of the completion of our financial closing procedures, final adjustments and other developments arising between now and the time that our financial results for the three months and year ended September 30, 2016 are finalized. The preliminary financial data included herein have been prepared by, and are the responsibility of, management. PricewaterhouseCoopers LLP, our independent registered public accounting firm, has not audited, reviewed, compiled or performed any procedures with respect to such preliminary estimates. Accordingly, PricewaterhouseCoopers LLP does not express an opinion or any other form of assurance with respect thereto.

Our management will have broad discretion in the use of the net proceeds from this offering and may allocate the net proceeds from this offering in ways that you and other stockholders may not approve.

Our management will have broad discretion in the use of the net proceeds, including for any of the purposes described in the section entitled “Use of Proceeds,” and you will not have the opportunity as part of your investment decision to assess whether the net proceeds are being used in ways with which you may not agree or may not otherwise be considered appropriate. Because of the number and variability of factors that will determine our use of the net proceeds from this offering, their ultimate use may vary substantially from their currently intended use. The failure of our management to use these funds effectively could harm our business. Pending their use, we may invest the net proceeds from this offering in short-term, investment-grade, interest-bearing securities. These investments may not yield a favorable return to our stockholders.

We may be unable to invest a significant portion of the net proceeds of this offering on acceptable terms.

Delays in investing the net proceeds of this offering or redeploying amounts repaid under the Credit Facility may impair our performance. We cannot assure you that we will be able to identify investments that meet our investment objectives or that any investment we make will produce a positive return. We may be unable to invest the net proceeds of this offering on acceptable terms within the time period that we anticipate or at all, which could adversely affect our financial condition and operating results.

Market interest rates may have an effect on the value of our common stock.

One of the factors that will influence the price of our common stock will be the distribution yield on our common stock (as a percentage of the price of our common stock) relative to market interest rates. An increase in market interest rates, which are currently at low levels relative to historical rates, may lead prospective purchasers of our common stock to expect a higher distribution yield and higher interest rates would likely increase our borrowing costs and potentially decrease funds available for distribution. Thus, higher market interest rates could cause the market price of our common stock to decrease.

Our NAV may change significantly since our last valuation at June 30, 2016.

Generally, our Board of Directors reviews and approves the fair value of our portfolio of investments on a quarterly basis. The last such quarterly review occurred as of June 30, 2016. Further, our financial statements

S-12

Table of Contents

have not been audited by our independent registered public accounting firm for any periods since September 30, 2015. The fair value of various individual investments in our portfolio and/or the aggregate fair value of our investments may have changed significantly since that time. We are currently in the process of determining the fair value of our portfolio as of September 30, 2016 and based on the preliminary assessment of our Board of Directors, the fair value has increased since June 30, 2016. If our Board of Directors makes a final determination that the fair value of our investment portfolio at September 30, 2016 was less than such fair value at June 30, 2016, then we will record an unrealized loss on our investment portfolio and report a lower NAV per share than is reflected in the Consolidated Selected Financial Data and the financial statements included elsewhere in this prospectus supplement. If our Board of Directors determines that the fair value of our investment portfolio at September 30, 2016 was greater than such fair value at June 30, 2016, we will record an unrealized gain on our investment portfolio and report a greater NAV per share than so reflected elsewhere in this prospectus supplement. Upon publication of this information in connection with our announcement of operating results for our quarter and fiscal year ended September 30, 2016, the market price of our common stock may fluctuate materially, and may be substantially less than the price per share you pay for our common stock in this offering.

Shares of closed-end investment companies, including BDCs, frequently trade at a discount to their NAV.

Shares of closed-end investment companies, including BDCs, frequently trade at a discount from NAV. This characteristic of closed-end investment companies and BDCs is separate and distinct from the risk that our NAV per share may decline. We cannot predict whether our common stock will trade at, above or below NAV, however our common stock has generally traded below NAV in the last three years. In addition, if our common stock trades below NAV, we will generally not be able to issue additional common stock at the market price without the approval of our stockholders and Board of Directors, including a majority of our independent directors. At our Annual Stockholders Meeting on February 11, 2016, our stockholders voted to allow us to issue common stock at a price below NAV per share for a one-year period. Our stockholders did not specify a maximum discount below NAV at which we are able to issue our common stock but we are unable to issue and sell more than 25% of our then outstanding common stock immediately prior to any offering below NAV. For instance, we have in the past and may in the future issue shares of our common stock below NAV.

Stockholders who do not participate in this offering may experience immediate dilution in an amount that may be material.

We have obtained approval from our stockholders for us to be able to sell an unlimited number of shares of our common stock at any level of discount from NAV per share in certain circumstances during a one-year period ending in February 2017 (subject to a limitation on issuing and selling in a single offering more than 25% of the shares of our common stock outstanding immediately prior to such offering). If we issue or sell shares of our common stock at a discount to NAV, and we may do so in this offering, it will pose a risk of dilution to our existing stockholders. In particular, stockholders who do not purchase additional shares at or below the discounted price in proportion to their current ownership will experience an immediate decrease in NAV per share (as well as in the aggregate NAV of their shares if they do not participate at all). These stockholders will also experience a disproportionately greater decrease in their participation in our earnings and assets and their voting power than the increase we experience in our assets, potential earning power and voting interests from such issuance or sale. In addition, such sales may adversely affect the price at which our common stock trades. For additional information and hypothetical examples of these risks, see “Sales of Common Stock Below Net Asset Value” in this prospectus supplement and in the accompanying prospectus.

Holders of our preferred stock and future holders of any securities ranking senior to our common stock have dividend, distribution and liquidation rights that are senior to the rights of the holders of our common stock.

In May 2014, we completed a public offering of the Series 2021 Term Preferred Stock, at a public offering price of $25.00 per share. In such offering, we issued 2.4 million shares of Series 2021 Term Preferred Stock. The shares of Series 2021 Term Preferred Stock have dividend, distribution and liquidation rights that are senior to

S-13

Table of Contents

the rights of the holders of our common stock. Further, in the future, we may attempt to increase our capital resources by making additional offerings of preferred equity securities or issuing debt securities. Upon liquidation, holders of our preferred stock, holders of our debt securities, if any, and lenders with respect to other borrowings, including the Credit Facility, would receive a distribution of our available assets in full prior to the holders of our common stock. Because our decision to issue securities in any future offering will depend on market conditions and other factors beyond our control, we cannot predict or estimate the amount, timing or nature of our future offerings. Thus, our common stockholders bear the risk of our future offerings reducing the per share trading price of our common stock and diluting their interest in us.

We may not be permitted to declare a dividend or make any distribution to stockholders or repurchase shares until such time as we satisfy the asset coverage tests under the provisions of the 1940 Act that apply to BDCs. As a BDC, we have the ability to issue senior securities only in amounts such that our asset coverage, as defined in the 1940 Act, equals at least 200% after each issuance of senior securities. If the value of our assets declines, we may be unable to satisfy this test. If that happens, we may be required to sell a portion of our investments and, depending on the nature of our leverage, repay a portion of our debt at a time when such sales and/or repayments may be disadvantageous.

Regulations governing our operation as a BDC and RIC will affect our ability to raise, and the way in which we raise, additional capital or borrow for investment purposes, which may have a negative effect on our growth. As a result of the annual distribution requirement to qualify as a RIC, we may need to periodically access the capital markets to raise cash to fund new investments. We may issue “senior securities,” including borrowing money from banks or other financial institutions only in amounts such that our asset coverage, as defined in the 1940 Act, equals at least 200% after such incurrence or issuance. Further, we may not be permitted to declare a dividend or make any distribution to our outstanding stockholders or repurchase shares until such time as we satisfy this test. Our ability to issue different types of securities is also limited. Compliance with these requirements may unfavorably limit our investment opportunities and reduce our ability in comparison to other companies to profit from favorable spreads between the rates at which we can borrow and the rates at which we can lend. As a BDC, therefore, we intend to continuously issue equity at a rate more frequent than our privately owned competitors, which may lead to greater stockholder dilution. We have incurred leverage to generate capital to make additional investments. If the value of our assets declines, we may be unable to satisfy the asset coverage test under the 1940 Act, which could prohibit us from paying distributions and could prevent us from qualifying as a RIC. If we cannot satisfy the asset coverage test, we may be required to sell a portion of our investments and, depending on the nature of our debt financing, repay a portion of our indebtedness at a time when such sales and repayments may be disadvantageous.

S-14

Table of Contents

SPECIAL NOTE REGARDING FORWARD-LOOKING STATEMENTS

All statements contained in this prospectus supplement or the accompanying prospectus, other than historical facts, may constitute “forward-looking statements.” These statements may relate to, among other things, future events or our future performance or financial condition. In some cases, you can identify forward-looking statements by terminology such as “may,” “might,” “believe,” “will,” “provided,” “anticipate,” “future,” “could,” “growth,” “plan,” “intend,” “expect,” “should,” “would,” “if,” “seek,” “possible,” “potential,” “likely” or the negative of such terms or comparable terminology. These forward-looking statements involve known and unknown risks, uncertainties and other factors that may cause our actual results, levels of activity, performance, financial condition or achievements to be materially different from any future results, levels of activity, performance or achievements expressed or implied by such forward-looking statements. Such factors include, but are not limited to:

We caution readers not to place undue reliance on any such forward-looking statements, which speak only as of the date made. We undertake no obligation to publicly update or revise any forward-looking statements, whether as a result of new information, future events or otherwise, after the date of this prospectus supplement or the accompanying prospectus, except as otherwise required by applicable law. The forward-looking statements contained in this prospectus supplement and the accompanying prospectus are excluded from the safe harbor protection provided by the Private Securities Litigation Reform Act of 1995 and Section 27A of the Securities Act of 1933, as amended (the “Securities Act”).

S-15

Table of Contents

USE OF PROCEEDS

We estimate that the net proceeds from the sale of the 2,000,000 shares of our common stock that we are offering, after deducting underwriting discounts and commissions and expenses of this offering payable by us, will be approximately $15.1 million (or $17.4 million, if the underwriters exercise their overallotment option in full) based on a public offering price of $7.98 per share.

We intend to use the net proceeds from this offering to help repay borrowings under the Credit Facility and for other general corporate purposes. Amounts repaid under the Credit Facility remain available for future borrowings and we may use the proceeds of future borrowings under the Credit Facility to make investments in accordance with our investment strategy and for other general corporate purposes. As of June 30, 2016, we had $73.3 million of borrowings outstanding under our Credit Facility. We currently have $65.7 million outstanding under the Credit Facility. Indebtedness under our Credit Facility currently accrues interest at the rate of 30-day LIBOR plus 3.25% (or 3.72% as of June 30, 2016) and the revolving period ends in January 2019 and is due and payable on or before April 19, 2020. We anticipate that substantially all of the net proceeds of the offering will be utilized in the manner described above within three months of the completion of this offering. Pending such utilization, we intend to invest the net proceeds of the offering primarily in cash, cash equivalents, U.S. government securities, and other high-quality debt investments that mature in one year or less from the date of investment, consistent with the requirements for continued qualification as a RIC for federal income tax purposes.

S-16

Table of Contents

CAPITALIZATION

The following table sets forth our capitalization as of June 30, 2016:

The following are our outstanding classes of securities as of October 24, 2016:

S-17

Table of Contents

SALES OF COMMON STOCK BELOW NET ASSET VALUE

At our 2016 annual stockholders meeting on February 11, 2016, our stockholders approved our ability to issue and sell shares of our common stock at a price below the then current NAV per common share during a period beginning on February 11, 2016 and expiring on the first anniversary of such date (the “Stockholder Approval”). The offering of common stock being made pursuant to this prospectus supplement will be at a price below our most recently estimated NAV per share range on October 24, 2016 of $8.05 to $8.15 per share. To sell shares of common stock at a price below NAV per share, pursuant to the Stockholder Approval, the 1940 Act mandates that a majority of our directors who have no financial interest in the sale and a majority of our independent directors have determined (i) that such sale and issuance is in our best interests and in the best interests of our stockholders and (ii) as of a time either immediately prior to the first solicitation by us or on our behalf of firm commitments to purchase such shares, or immediately prior to the issuance of such shares, and in good faith and in consultation with the underwriters of the offering, that the price at which such shares of common stock are to be sold is not less than a price which closely approximates the market value of those shares of common stock, less any distributing commission or discount.

In addition to the mandates of the 1940 Act pertaining to issuances and sales of common stock at a price below NAV per share, our Stockholder Approval requires that in any offering of common stock at a price below NAV per share the total number of shares issued and sold pursuant to such Stockholder Approval may not exceed 25% of our currently outstanding common stock immediately prior to each such sale. This offering meets this additional requirement.

This offering of common stock will be conducted below its NAV per share and is designed to raise capital to help repay outstanding borrowings under the Credit Facility and for other general corporate purposes.

In making a determination that an offering of common stock below its NAV per share is in our and our stockholders’ best interests, our Board of Directors has considered a variety of factors including:

Our Board of Directors has also considered the fact that sales of shares of common stock at a discount will benefit the Adviser as the Adviser will ultimately earn additional investment management fees on the proceeds of such offerings, as it would from the offering of any other securities of the Company or from the offering of common stock at a premium to NAV per share.

S-18

Table of Contents

We will not sell shares of our common stock under this prospectus pursuant to the Stockholder Approval without first filing a new post-effective amendment to the registration statement if the cumulative dilution to our net asset value per share from offerings under the registration statement, as amended by any post-effective amendments, exceeds 15%. This would be measured separately for each offering pursuant to the registration statement, as amended, by calculating the percentage dilution or accretion to aggregate net asset value from that offering and then summing the percentage from each offering. We do not expect dilution from this offering to exceed 15%.

In addition, the maximum number of shares issuable below NAV per share that could result in such dilution is limited to 25% of our then outstanding common stock immediately prior to each such offering. Sales by us of our common stock at a discount from NAV per share pose potential risks for our existing stockholders whether or not they participate in this offering, as well as for new investors who participate in this offering. Any sale of common stock at a price below NAV per share results in an immediate dilution to existing common stockholders who do not participate in such sale on at least a pro-rata basis. See “Risk Factors—Stockholders who do not participate in this offering will experience immediate dilution in an amount that may be material” in this prospectus supplement and “Risk Factors—Risks Related to an Investment in Our Securities” in the accompanying prospectus.

The following three headings and accompanying tables explain and provide hypothetical examples on the impact of this offering of our common stock at a price less than NAV per share on three different types of investors:

Impact on Existing Stockholders Who Do Not Participate in the Offering

Our existing common stockholders who do not participate in this offering or who do not buy additional shares in the secondary market at the same or lower price we obtain in this offering (after expenses and commissions) face the greatest potential risks. These stockholders will experience an immediate dilution in the NAV of the common shares they hold and their NAV per common share. These common stockholders will also experience a disproportionately greater decrease in their participation in our earnings and assets and their voting power than the increase we will experience in our assets, potential earning power and voting interests due to this offering. These stockholders may also experience a decline in the market price of their shares, which often reflects to some degree announced or potential increases and decreases in NAV per common share. This decrease could be more pronounced as the size of the offering and level of discounts increase.

S-19

Table of Contents

The following table illustrates the level of NAV dilution that could be experienced by an existing common stockholder that does not participate in this offering. It is not possible to predict the level of market price decline that may occur. The table below is based upon financial information as of June 30, 2016 except NAV per share, which is based on the midpoint of the estimated range of NAV per share as of October 24, 2016. The following example assumes a sale of 2,000,000 shares of common stock at a public offering price of $7.98 per share with a 4.00% underwriting discount and commission and $240,000 of estimated offering expenses ($0.12 per share, net). The numbers in this table have been rounded to the nearest hundredth of one percent.

Impact on Existing Stockholders Who Do Participate in the Offering

Our existing common stockholders who participate in this offering or who buy additional shares in the secondary market at the same or lower price as we obtain in the offering (after expenses and commissions) will experience the same types of NAV dilution as the nonparticipating common stockholders, albeit at a lower level, to the extent they purchase less than the same percentage of the discounted offering as their interest in our common shares immediately prior to the offering. The level of NAV dilution will decrease as the number of common shares such stockholders purchase increases. Existing common stockholders who buy more than such percentage will experience NAV dilution but will, in contrast to existing common stockholders who purchase less than their proportionate share of this offering, experience accretion in NAV per common share over their investment per share and will also experience a disproportionately greater increase in their participation in our earnings and assets and their voting power than our increase in assets, potential earning power and voting interests due to this offering. The level of accretion will increase as the excess number of shares such common stockholder purchases increases. Even a common stockholder who over-participates will, however, be subject to the risk that we may make additional discounted offerings in which such common stockholder does not participate, in which case such a stockholder will experience NAV dilution as described above in such subsequent offerings. These stockholders may also experience a decline in the market price of their shares, which often reflects to some degree announced

S-20

Table of Contents

or potential increases and decreases in NAV per share. This decrease could be more pronounced as the size of the offering and level of discount to NAV increases.

The following table illustrates the level of dilution and accretion for a common stockholder that acquires shares equal to (1) 50% of its proportionate share of the offering (i.e., 1,000 shares, which is 0.05% of the offering rather than its 0.10% proportionate share) and (2) 150% of such percentage (i.e., 3,000 shares, which is 0.15% of the offering rather than its 0.10% proportionate share). The table below is shown based upon financial information as of June 30, 2016 except NAV per share, which is based on the midpoint of the estimated range of NAV per share as of October 24, 2016. The following example assumes a sale of 2,000,000 shares of common stock at a public offering price of $7.98 per share, with a 4.00% underwriting discount and commission and $240,000 of estimated offering expenses ($0.12 per share, net). The numbers in this table have been rounded to the nearest hundredth of one percent.

S-21

Table of Contents

Impact on New Investors

Investors who are not currently stockholders, but who participate in an offering below NAV and whose investment per common share is greater than the resulting NAV per share (due to selling compensation and expenses paid by us) will experience an immediate decrease in the NAV of their shares and their NAV per share compared to the price they pay for their shares of common stock. Investors who are not currently stockholders and who participate in this offering and whose investment per common share is also less than the resulting NAV per common share due to selling compensation and expenses paid by the issuer being significantly less than the discount per common share will experience an immediate increase in the NAV of their shares and their NAV per share compared to the price they pay for their shares of common stock. These investors will experience a disproportionately greater participation in our earnings and assets and their voting power than our increase in assets, potential earning power and voting interests. These investors will, however, be subject to the risk that we may make additional discounted offerings in which such new common stockholder does not participate, in which case such new stockholder will experience dilution as described above in such subsequent offerings. These investors may also experience a decline in the market price of their shares of, which often reflects to some degree announced or potential increases and decreases in NAV per share. This decrease could be more pronounced as the size of the offering and level of discounts increases.

The following table illustrates the level of dilution or accretion for new investors that would be experienced by a new investor in the same percentage (0.10%) of the common shares in the offering as the common stockholder in the prior examples held immediately prior to the offering. These stockholders may also experience a decline in the market price of their shares of common stock, which often reflects to some degree announced or potential increases and decreases in NAV per share. This decrease could be more pronounced as the size of the offering and level of discount to NAV increases. It is not possible to predict the level of market price decline that may occur. The table below is shown based upon financial information as of June 30, 2016 except NAV per share, which is based on the midpoint of the estimated range of NAV per share as of October 24, 2016. The following example assumes a sale of 2,000,000 shares of common stock at a public offering price of $7.98 per share, with a 4.00% underwriting discount and commission and $240,000 of estimated offering expenses ($0.12 per share, net). The numbers in this table have been rounded to the nearest hundredth of one percent.

S-22

Table of Contents

DISTRIBUTIONS

We currently intend to distribute in the form of cash distributions a minimum of 90% of our ordinary income and net short-term capital gains in excess of net long-term capital losses, if any, on a quarterly basis to our stockholders in the form of monthly distributions. We intend to retain net long-term capital gains in excess of net short-term losses and treat them as deemed distributions for tax purposes. We report the estimated tax characteristics of each distribution when declared while the actual tax characteristics of distributions are reported annually to each stockholder on IRS Form 1099-DIV. There is no assurance that we will maintain our status as a RIC or achieve investment results necessary for any specified level of cash distributions or year-to-year increases in cash distributions. At the option of a holder of record of common stock, all cash distributions can be reinvested automatically under our distribution reinvestment plan in additional whole and fractional shares. A stockholder whose shares are held in the name of a broker or other nominee should contact the broker or nominee regarding participation in our distribution reinvestment plan on the stockholder’s behalf. See “Risk Factors—Risks Related to Our Regulation and Structure—We will be subject to corporate-level tax if we are unable to satisfy Code requirements for RIC qualification;” “Dividend Reinvestment Plan;” and “Material U.S. Federal Income Tax Considerations” in the accompanying prospectus.

SHARE PRICE DATA

Our common stock is traded on the NASDAQ under the symbol “GLAD.” The following table reflects, by quarter, the high and low sales prices per share of our common stock on the NASDAQ, the high and low sales prices as a percentage of NAV per share and quarterly distributions declared per share for each quarter since October 1, 2013.

S-23

Table of Contents

As of October 24, 2016, there were 42 record owners of our common stock. The last reported sales price of our common stock on NASDAQ on October 25, 2016 was $8.19 per share, representing a 1.1% premium to our estimated NAV per share of $8.10, which is based on the midpoint of the estimated range of NAV per share as of October 24, 2016.

S-24

Table of Contents

CONSOLIDATED SELECTED FINANCIAL DATA

The following consolidated selected financial data for the fiscal years ended September 30, 2015, 2014, 2013, 2012 and 2011 are derived from our audited consolidated financial statements. The consolidated selected financial data for the nine months ended June 30, 2016 and 2015 are derived from our unaudited consolidated financial statements included in this prospectus supplement. The “other unaudited data” included at the bottom of the table is also unaudited. The data should be read in conjunction with our consolidated financial statements and notes thereto and “Management’s Discussion and Analysis of Financial Condition and Results of Operations” included elsewhere in this prospectus supplement.

S-25

Table of Contents

S-26

Table of Contents

SELECTED QUARTERLY FINANCIAL DATA

The following tables set forth certain quarterly financial information for each of the eight quarters in the two fiscal years ended September 30, 2015 and September 30, 2014 and the first three quarters of the fiscal year ending September 30, 2016. The information was derived from our unaudited consolidated financial statements. Results for any quarter are not necessarily indicative of results for the entire fiscal year or for any future quarter.

S-27

Table of Contents

MANAGEMENT’S DISCUSSION AND ANALYSIS OF

FINANCIAL CONDITION AND RESULTS OF OPERATIONS

You should read the following analysis of our financial condition and results of operations in conjunction with our condensed consolidated financial statements and the related notes contained elsewhere in this prospectus supplement and in the accompanying prospectus. Historical financial condition and results of operations and percentage relationships among any amounts in the financial statements are not necessarily indicative of financial condition, results of operations or percentage relationships for any future periods. Except per share amounts, dollar amounts in the tables included herein are in thousands unless otherwise indicated.

OVERVIEW

General

We were incorporated under the Maryland General Corporation Law on May 30, 2001. We were established for the purpose of investing in debt and equity securities of established private businesses in the U.S. We operate as an externally managed, closed-end, non-diversified management investment company, and have elected to be treated as a business development company (“BDC”) under the Investment Company Act of 1940, as amended (the “1940 Act”). In addition, for federal income tax purposes we have elected to be treated as a regulated investment company (“RIC”) under the Internal Revenue Code of 1986, as amended (the “Code”). As a BDC and RIC, we are subject to certain constraints, including limitations imposed by the 1940 Act and the Code.

Our investment objectives are to: (1) achieve and grow current income by investing in debt securities of established businesses that we believe will provide stable earnings and cash flow to pay expenses, make principal and interest payments on our outstanding indebtedness and make distributions to stockholders that grow over time; and (2) provide our stockholders with long-term capital appreciation in the value of our assets by investing in equity securities of established businesses that we believe can grow over time to permit us to sell our equity investments for capital gains. To achieve our objectives, our primary investment strategy is to invest in several categories of debt and equity securities, with each investment generally ranging from $8 million to $30 million, although investment size may vary, depending upon our total assets or available capital at the time of investment. We intend for our investment portfolio to consist of approximately 90.0% debt investments and 10.0% equity investments, at cost. As of June 30, 2016, our investment portfolio was made up of approximately 90.2% debt investments and 9.8% equity investments, at cost.

We focus on investing in small and medium-sized middle market private businesses in the U.S. that meet certain criteria, including, but not limited to, the following: the sustainability of the business’ free cash flow and its ability to grow it over time, adequate assets for loan collateral, experienced management teams with a significant ownership interest in the business, reasonable capitalization of the borrower, including an ample equity contribution or cushion based on prevailing enterprise valuation multiples and, to a lesser extent, the potential to realize appreciation and gain liquidity in our equity position, if any. We lend to borrowers that need funds for growth capital, to finance acquisitions, or recapitalize or refinance their existing debt facilities. We typically avoid investing in high-risk, early-stage enterprises. Our targeted portfolio companies are generally considered too small for the larger capital marketplace. We invest by ourselves or jointly with other funds or management of the portfolio company, depending on the opportunity. If we are participating in an investment with one or more co-investors, our investment is likely to be smaller than if we were investing alone.

In July 2012, the SEC granted us an exemptive order that expanded our ability, under certain circumstances, to co-invest with Gladstone Investment Corporation (“Gladstone Investment”) and any future BDC or closed-end management investment company that is advised (or sub-advised if it controls the fund) by the Adviser or any combination of the foregoing subject to the conditions in the SEC’s order. We believe this has enhanced and will continue to enhance our ability to further our investment objectives and strategies.

S-28

Table of Contents

We are externally managed by Gladstone Management Corporation (the “Adviser”), an investment adviser registered with the SEC and an affiliate of ours, pursuant to an investment advisory and management agreement (the “Advisory Agreement”). The Adviser manages our investment activities. We have also entered into an administration agreement (the “Administration Agreement”) with Gladstone Administration, LLC (the “Administrator”), an affiliate of ours and the Adviser, whereby we pay separately for administrative services.

Additionally, since February 2011, Gladstone Securities, LLC (“Gladstone Securities”), a privately-held broker-dealer registered with the Financial Industry Regulatory Authority and insured by the Securities Investor Protection Corporation, which is 100% indirectly owned and controlled by Mr. Gladstone, our chairman and chief executive officer, has provided other services, such as investment banking and due diligence services, to certain of our portfolio companies, for which Gladstone Securities receives a fee.

Our shares of common stock and 6.75% Series 2021 Term Preferred Stock (our “Series 2021 Term Preferred Stock”) are traded on the NASDAQ Global Select Market (“NASDAQ”) under the trading symbols “GLAD” and “GLADO,” respectively.

Business

Portfolio and Investment Activity

During the nine months ended June 30, 2016, we invested $54.3 million in five new portfolio companies and extended $5.6 million of investments to existing portfolio companies. In addition, during the nine months ended June 30, 2016, we exited 10 portfolio companies through sales and early payoffs. We received a total of $98.4 million in combined net proceeds and principal repayments from the aforementioned portfolio company exits as well as existing portfolio companies during the nine months ended June 30, 2016. This activity resulted in a net reduction in our overall portfolio of five portfolio companies to 43 and a net decrease of 5.8% in our portfolio at cost since September 30, 2015. Our continued focus throughout 2016 will be to rebuild our investment portfolio by making new investments and to exit challenged and non-strategic investments in our portfolio in an orderly manner over the next several quarters. Since our initial public offering in August 2001, we have made 423 different loans to, or investments in, 201 companies for a total of approximately $1.5 billion, before giving effect to principal repayments on investments and divestitures.

During the nine months ended June 30, 2016, the following significant transactions occurred:

S-29

Table of Contents

Refer to Note 13—Subsequent Events in the accompanying Consolidated Financial Statements included elsewhere in this prospectus supplement for portfolio activity occurring subsequent to June 30, 2016.

Capital Raising

We issued shares of our common stock in an overnight offering in October 2015 with the overallotment closing in November 2015 at a public offering price of $8.55 per share, which was below the then current net asset value (“NAV”) of $9.06 per share. The resulting proceeds, in part, provided us with additional equity capital to help ensure continued compliance with regulatory tests and will allow us to grow the portfolio and generate additional income through new investments. Refer to “Liquidity and Capital Resources — Equity — Common Stock” for further discussion of our common stock offerings.

Although we were able to access the capital markets over the last year, uncertain market conditions continue to affect the trading price of our capital stock and thus may inhibit our ability to finance new investments through the issuance of equity. The current volatility in the credit market and the uncertainty surrounding the U.S. economy have led to significant stock market fluctuations, particularly with respect to the stock of financial services companies like ours. During times of increased price volatility, our common stock may be more likely to continue to trade at a price below our NAV per share, which is not uncommon for BDCs like us.

On August 2, 2016, the closing market price of our common stock was $7.91, a 0.5% discount to our June 30, 2016 NAV per share of $7.95. When our stock trades below NAV per common share, as it has consistently traded over the last several years, our ability to issue equity is constrained by provisions of the 1940 Act, which generally prohibits the issuance and sale of our common stock below NAV per common share without stockholder approval, other than through sales to our then-existing stockholders pursuant to a rights offering. At our annual meeting of stockholders held on February 11, 2016, our stockholders approved a proposal which authorizes us to sell shares of our common stock at a price below our then current NAV per common share subject to certain limitations (including, but not limited to, that the number of shares issued and sold pursuant to such authority does not exceed 25.0% of our then outstanding common stock immediately prior to each such sale) for a period of one year from the date of approval, provided that our board of directors (our “Board of Directors”) makes certain determinations prior to any such sale.

S-30

Table of Contents

Regulatory Compliance

Challenges in the current market are intensified for us by certain regulatory limitations under the Code and the 1940 Act that may further constrain our ability to access the capital markets. To qualify to be taxed as a RIC, we must distribute on an annual basis at least 90.0% of our investment company taxable income, which is generally our net ordinary income plus the excess of our net short-term capital gains over net long-term capital losses. Our ability to seek external debt financing, to the extent that it is available under current market conditions, is further subject to the asset coverage limitations of the 1940 Act that require us to have an asset coverage ratio (as defined in Section 18 of the 1940 Act) of at least 200% on our “senior securities representing indebtedness” and our “senior securities that are stock.”

We expect that, given these regulatory and contractual constraints in combination with current market conditions, the debt and equity capital available may be limited in the near term. However, we believe that the recent amendments to our Credit Facility to decrease the interest rate on advances and extend its revolving period end date until 2019, and our syndication and expansion of our Credit Facility in June 2015 has increased our ability to make investments in middle market businesses that we believe will help us achieve attractive long-term returns for our stockholders.

Recent Developments

Registration Statement

We filed a universal shelf registration statement (our “Registration Statement”) on Form N-2 (File No. 333-208637) with the SEC on December 18, 2015, and subsequently filed Pre-Effective Amendment No. 1 on March 17, 2016 and Pre-Effective Amendment No. 2 on March 29, 2016, which the SEC declared effective on March 29, 2016. Our Registration Statement registered an aggregate of $300.0 million in securities, consisting of common stock, preferred stock, subscription rights, debt securities and warrants to purchase common stock, preferred stock or debt securities. No securities have been issued under the registration statement as of June 30, 2016.

Common Stock Share Repurchase Program

In January 2016, our Board of Directors authorized a share repurchase program for up to an aggregate of $7.5 million of the Company’s common stock. The repurchases are intended to be implemented through open market transactions on U.S. exchanges or in privately negotiated transactions, in accordance with applicable securities laws, and any market purchases will be made during open trading window periods or pursuant to any applicable Rule 10b5-1 trading plans. The timing, prices, and amounts of repurchases will depend upon prevailing market prices, general economic and market conditions and other considerations. The repurchase program does not obligate us to acquire any particular number of shares of common stock. The termination date is the earlier of repurchasing the total authorized amount of $7.5 million or January 31, 2017. During the three months ended June 30, 2016, we repurchased 41,414 shares of our common stock at an average share price of $6.95, resulting in gross purchases of $0.3 million. During the nine months ended June 30, 2016, we repurchased 87,200 shares of our common stock at an average share price of $6.53, resulting in gross purchases of $0.6 million.

S-31

Table of Contents

RESULTS OF OPERATIONS

Comparison of the Three Months Ended June 30, 2016, to the Three Months Ended June 30, 2015

NM = Not Meaningful

Investment Income

Total investment income decreased slightly by 0.9% for the three months ended June 30, 2016, as compared to the prior year period. This decrease was primarily due to a decrease in interest income, which resulted from a decrease in the size of our interest-bearing portfolio for the three months ended June 30, 2016 as compared to the prior year period, partially offset by an increase in other income.

Interest income, net decreased by 9.4% for the three months ended June 30, 2016, as compared to the prior year period. The level of interest income from investments is directly related to the principal balance of our interest-bearing investment portfolio outstanding during the period multiplied by the weighted average yield. The weighted average principal balance of our interest-bearing investment portfolio during the three months ended June 30, 2016, was $303.6 million, compared to $330.6 million for the prior year period, a decrease of 8.2%. This decrease was due primarily to exits that occurred during the first quarter of fiscal year 2016. The weighted average yield on our interest-bearing investment portfolio is based on the current stated interest rate on

S-32

Table of Contents

interest-bearing investments which decreased to 10.9% for the three months ended June 30, 2016, compared to 11.1% for the three months ended June 30, 2015, inclusive of any allowances on interest receivables made during those periods.

As of June 30, 2016, two portfolio companies were either fully or partially on non-accrual status, with an aggregate debt cost basis of approximately $26.5 million, or 7.5% of the cost basis of all debt investments in our portfolio. As of June 30, 2015, four portfolio companies were either fully or partially on non-accrual status, with an aggregate debt cost basis of approximately $49.2 million, or 13.4%, of the cost basis of all debt investments in our portfolio.

For the three months ended June 30, 2016, other income increased by 92.1% as compared to the prior year period. For the three months ended June 30, 2016, other income consisted primarily of $1.5 million in success fees recognized and $0.1 million in prepayment fees received. Other income for the three months ended June 30, 2015, consisted primarily of $0.5 million in dividend income received and $0.3 million in success fees recognized.

The following tables list the investment income for our five largest portfolio company investments at fair value during the respective periods:

Expenses

Expenses, net of any voluntary, irrevocable and non-contractual credits to fees from the Adviser, decreased by 3.2% for the three months ended June 30, 2016, as compared to the prior year period. This decrease was primarily due to a decrease in interest expense on borrowings and a decrease in net base management fees, partially offset by an increase in net incentive fee and other expenses.

S-33

Table of Contents

Interest expense on borrowings decreased by $0.4 million, or 37.3%, during the three months ended June 30, 2016, as compared to the prior year period, due primarily to a decrease in the borrowings outstanding under our Credit Facility during the period due to the sales and payoffs discussed above. The weighted average balance outstanding under our Credit Facility during the three months ended June 30, 2016, was $52.5 million, as compared to $109.8 million in the prior year period, a decrease of 52.2%.

Net base management fee earned by the Adviser decreased by $0.7 million, or 14.3%, during the three months ended June 30, 2016, as compared to the prior year period, resulting from a decrease in the average total assets outstanding. Our Board of Directors accepted an unconditional, non-contractual and irrevocable voluntary credit of $0.2 million from the Adviser to reduce the income-based incentive fee to the extent net investment income for the quarter ended June 30, 2016 did not cover 100.0% of the distributions to common stockholders during the period. The credit granted for the quarter ended June 30, 2015, was $0.8 million.

The base management, loan servicing and incentive fees, and associated unconditional, non-contractual, and irrevocable voluntary credits, are computed quarterly, as described under “Transactions with the Adviser” in Note 4—Related Party Transactions of the notes accompanying our Consolidated Financial Statements included elsewhere in this prospectus supplement and are summarized in the following table:

S-34

Table of Contents

Net Realized and Unrealized Gain (Loss)

Net Realized Gain (Loss) on Investments

We had no significant realized gains (losses) on investments for the three months ended June 30, 2016. For the three months ended June 30, 2015, we recorded a net realized loss on investments of $1.1 million, which resulted primarily from the exit of our investment in Sunburst during the three months ended June 30, 2015.

Net Unrealized Appreciation (Depreciation) of Investments

During the three months ended June 30, 2016, we recorded net unrealized appreciation of investments of $0.7 million. The realized gain (loss) and unrealized appreciation (depreciation) across our investments for the three months ended June 30, 2016, were as follows: