$250,000,000GOLUB CAPITAL BDC,INC.Common Stock We are an externally managed, closed-end, non-diversified management investment company that has electedto be regulated as a business development company under the Investment Company Act of 1940, as amended, orthe 1940 Act. Our investment objective is to generate current income and capital appreciation by investingprimarily in one stop (a loan that combines characteristics of traditional first lien senior secured loans and secondlien or subordinated loans and that are often referred to by other middle-market lenders as unitranche loans) andother senior secured loans of U.S. middle-market companies. We also selectively invest in second lien andsubordinated 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. GCAdvisors LLC and Golub Capital LLC are affiliated with Golub Capital (as defined herein), a leading lender toU.S. middle-market companies that has over $75.0 billion of capital under management as of April1, 2025. We, our investment adviser and our administrator have entered into an Equity Distribution Agreement, datedOctober6, 2023, as amended on December4, 2024 and May16, 2025 with Keefe, Bruyette& Woods,Inc. andRegions Securities LLC, who we refer to as the Sales Agents, relating to the shares of common stock offered bythis prospectus supplement and the accompanying prospectus. The Equity Distribution Agreement provides thatwe may offer and sell shares of our common stock having an aggregate offering price of up to $250.0 millionfrom time to time through the Sales Agents (which amount excludes the shares of common stock previously soldpursuant to the Equity Distribution Agreement prior to the date hereof). Sales of our common stock, if any, underthis prospectus supplement and the accompanying prospectus may be made in negotiated transactions ortransactions that are deemed to be “at the market,” as defined in Rule415 under the Securities Act of 1933, asamended, or the Securities Act, including sales made directly on the Nasdaq Global Select Market or similarsecurities exchanges or sales made to or through a market maker other than on an exchange, at prices related tothe prevailing market prices or at negotiated prices. Pursuant to the Equity Distribution Agreement, the Sales Agents will receive a commission from us of up to1.50% of the gross sales price. The Sales Agents are not required to sell any specific number or dollar amount ofcommon stock but each Sales Agent will use its commercially reasonable efforts consistent with its sales andtrading practices to sell the shares of our common stock offered by this prospectus supplement and theaccompanying prospectus. See “Plan of Distribution” beginning on pageS-16 of this prospectus supplement. Thesales price per share of our common stock offered by this prospectus supplement and the accompanyingprospectus, less commissions payable under the Equity Distribution Agreement and discounts, if any, will not beless than the net asset value per share of our common stock at the time of such sale. Our common stock is traded on the Nasdaq Global Select Market under the symbol “GBDC”. The lastreported closing price for our common stock on May15, 2025 was $14.94per share. The net asset value of ourcommon stock as of March31, 2025 (the last date prior to the date of this prospectus supplement on which wedetermined net asset value) was $15.04per share. Shares of closed-end investment companies, including business development companies, frequentlytrade at a discount to their net asset value. If our shares trade at a discount to our net asset value, it willlikely increase the risk of loss for purchasers in this offering. Investing in our common stock involves ahigh degree of risk. Before buying any of our common stock, you should read the discussion of the materialrisks of investing in our common stock, including the risk of leverage, in “Risk Factors” beginning onpage8of the accompanying prospectus or otherwise included in or incorporated by reference herein or inthe accompanying prospectus and in any free writing prospectuses we have authorized for use inconnection with this offering, and under similar headings in the other documents that are incorporated byreference into this prospectus supplement and the accompanying prospectus. This prospectus supplement and the accompanying prospectus contain important information you shouldknow before investing in our common stock. Please read this prospectus supplement and the accompanyingprospectus, and the documents incorporated by reference herein and therein, before you invest and keep it forfuture reference. We file annual, quarterly and current reports, proxy statements and other information about uswith the Securities and Exchange Commission, or the SEC. We maintain a website athttp://www.golubcapitalbdc.comand make all of our annual, qu