PROSPECTUS SUPPLEMENT (to Prospectus dated June 27, 2019) GOLUB CAPITAL BDC, INC. Up to 33,451,902 Shares of Common Stock Issuable Upon Exercise of Rights to Subscribe for Such Shares 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, or the 1940 Act. Our investment objective is to generate current income and capital appreciation by investing primarily in one stop and other senior secured loans of U.S. middle-market companies. We can also selectively invest in second lien and subordinated loans of, and warrants and minority equity securities in, U.S. middle-market companies. GC Advisors LLC serves as our investment adviser. Golub Capital LLC serves as our administrator. GC Advisors LLC and Golub Capital LLC are affiliated with Golub Capital (as defined herein), a leading lender to middle-market companies that has over $30.0 billion of capital under management as of December 31, 2019. We are issuing transferable subscription rights to our stockholders of record as of 5:00 p.m., New York City time, on April 8, 2020, entitling the holders thereof to subscribe for up to an aggregate of 33,451,902 shares of our common stock. Record date stockholders will receive one right for each four outstanding shares of common stock owned on the record date. The rights entitle the holders to purchase one new share of common stock for every right held. In addition, record date stockholders who fully exercise their rights will be entitled to subscribe, subject to the limitations described in this prospectus supplement and subject to allotment, for additional shares that remain unsubscribed as a result of any unexercised rights. Rights holders who exercise their rights will have no right to rescind their subscriptions after receipt of their completed subscription certificates together with payment for shares or a notice of guaranteed delivery by the subscription agent. Certain affiliates of Golub Capital LLC, an affiliate of GC Advisors, have indicated that they intend to over-subscribe and to make a total investment of up to $125 million in shares of our common stock pursuant to exercise of the primary subscription and/or the over-subscription privilege. As a result of the terms of this offering, stockholders who do not fully exercise their rights will own, upon completion of this offering, a smaller proportional interest in us than they owned prior to the offering. In addition, because the subscription price per share will likely be less than the net asset value per share, based on our current market price, the offering will likely result in an immediate dilution of net asset value per share for all of our stockholders. This offering will also cause dilution in the net investment income per each share of our common stock, which may affect the amount per share we are able to distribute subsequent to completion of the offering. Such dilution is not currently determinable because it is not known how many shares will be subscribed for or what the net asset value or market price of our common stock will be on the expiration date for the offer. If the subscription price per share is substantially less than the current net asset value per share, such dilution could be substantial. Any such dilution will disproportionately affect non-exercising stockholders. If the subscription price is less than our net asset value per share, then all stockholders will experience a decrease in the net asset value per share held by them, irrespective of whether they exercise all or any portion of their rights. See “Risk Factors — Your economic and voting interest in us, as well as your proportionate interest in our net asset value, could be diluted as a result of this rights offering” and “Dilution” in this prospectus supplement for more information. After giving effect to the sale of shares of our common stock in this offering, as of December 31, 2019, assuming all rights are exercised at the estimated subscription price of $11.45 per share and our receipt of the estimated net proceeds from that sale, our “as adjusted” net asset value would have been approximately $2.604 billion, or approximately $15.57 per share, representing immediate net asset value dilution of approximately $1.09 per share to our existing stockholders. Our common stock is traded on the Nasdaq Global Select Market under the symbol “GBDC”. The last reported closing price for our common stock on March 30, 2020 was $12.62 per share. The net asset value of our common stock as of December 31, 2019 (the last date prior to the date of this prospectus supplement on which we determined net asset value) was $16.66 per share. The subscription rights are transferable and we have applied to list the Rightsonthe Nasdaq Global Select Market under the symbol “GBDCR”. See “The Offering” for a complete discussion of the terms of this offering. The subscription price per share will be 92.5% of the volume-weighted average of the sales prices of our shares of common stock on the Nasdaq Global Select Market for the five consecutive trading days ending on the expiration date of the offering. Because the subscription price will be determined on the expiration date, rights holders will generally not know the subscription price at the time of exercise. The rights will expire if they are not exercised by 5:00 p.m., New York City time, on May 6, 2020, the expiration date of the offering, unless extended as described in this prospectus supplement. We, in our sole discretion, can extend the period for exercising the subscription rights. Shares of closed-end investment companies, including business development companies, frequently trade at a discount to their net asset value. If our shares continue to trade at a discount to our net asset value, it will likely increase the risk of loss for purchasers in this offering. Investing in our securities involves a high degree of risk. Before buying any securities, you should read the discussion of the material risks of investing in the rights and our common stock, including the risk of leverage, in “Risk Factors” beginning on page S-19 of this prospectus supplement and on page 8 of the accompanying prospectus or otherwise included in or incorporated by reference herein or the accompanying prospectus and in any free writing prospectuses we have authorized for use in connection with a specific offering, and under similar headings in the other documents that are incorporated by reference into this prospectus supplement and the accompanying prospectus. This prospectus supplement and the accompanying prospectus contain important information you should know before investing in the rights. Please read this prospectus supplement and the accompanying prospectus, and the documents incorporated by reference herein and therein, before you invest and keep it for future reference. We file annual, quarterly and current reports, proxy statements and other information about us with the Securities and Exchange Commission, or the SEC. We maintain a website at http://www.golubcapitalbdc.com and make all of our annual, quarterly and current reports, proxy statements and other publicly filed information available on or through our website. Information on our website is not incorporated into or a part of this prospectus supplement or the accompanying prospectus or any writing prospectus. You can also obtain such information, free of charge, and make shareholder inquiries by calling us collect at (212) 750-6060 or by contacting us at 200 Park Avenue, 25th Floor, New York, New York 10166, Attention: Investor Relations or [email protected]. The SEC also maintains a website at http://www.sec.gov that contains such information. We generally invest in securities that have been rated below investment grade by independent rating agencies or that would be rated below investment grade if they were rated. These securities, which are also referred to as “junk,” have predominantly speculative characteristics with respect to the issuer’s capacity to pay interest and repay principal. In addition, many of our debt investments have floating interest rates that reset on a periodic basis and typically do not fully pay down principal prior to maturity, which could increase our risk of losing part or all of our investment. Per Share Total(4) Estimated subscription price(1) ........................................................ $11.45 $383,024,278 Estimated sales load (underwriting discounts and commissions)(2)(3) ................................... $0.23 $ 7,693,937 Proceeds to us, before estimated expenses(1)(3) ................................................ $11.22 $375,330,340 (1) Estimated on the basis of 92.5% of the volume-weighted average of the sales prices of our shares of common stock on the Nasdaq Global Select Market for the five consecutive trading days ending on March 30, 2020. See “The Offering — Subscription Price.” (2) In connection with this offering, Wells Fargo Securities, LLC, the dealer manager for this offering, will receive a fee for certain financial advisory, marketing and soliciting services equal to 2% of the subscription price per share for each share issued other than any shares issued to pursuant to exercise of the primary subscription and/or the over-subscription privilege other than affiliates of Golub Capital LLC, an affiliate of GC Advisors, and 1% of the subscription price per share for each share issued to affiliates of Golub Capital LLC. The estimated sales load assumes all shares are purchased other than by affiliates of Golub Capital LLC. See “The Offering — Distribution Arrangements.” (3) We estimate that we will incur offering expenses of approximately $625,000 in connection with this offering. We estimate that net proceeds to us after expenses will be $374,705,340 assuming all of the rights are exercised at the estimated subscription price.