USS R-21 (SS-98) was an R-class coastal and harbor defense submarine of the United States Navy. Her keel was laid down on 19 April 1917 by the Lake Torpedo Boat Company in Bridgeport, Connecticut; the R-boats built by Lake Torpedo Boat (R-21 through R-27) are sometimes considered a separate class from those built by Fore River Shipbuilding (R-1 through R-14) and Union Iron Works (R-15 through R-20). She was launched on 10 July 1918 sponsored by Mrs. Dallas C. Laizure and commissioned on 17 June 1919 with Lieutenant Commander Morris D. Gilmore in command.