USS SC-21, until July 1920 known as USS Submarine Chaser No. 21 or USS S.C. 21, was an SC-1-class submarine chaser built for the United States Navy during World War I. SC-21 was a wooden-hulled 110-foot (34 m) submarine chaser built at the New York Navy Yard at Brooklyn, New York. She was commissioned on either 20 September or 19 October 1917 as USS Submarine Chaser No. 21, abbreviated at the time as USS S.C. 21, under the command of Lieutenant Alfred Y. Lanphier, who later would become the commander of the USS Patterson Group of submarine chasers.