The USS S-21 was a United States Navy submarine, part of the S-class fleet, launched on March 1, 1923. It was designed for coastal defense and had a displacement of about 1,000 tons when submerged. The submarine was equipped with torpedoes and had a crew of around 40 men, allowing it to operate effectively in shallow waters. During its service, the USS S-21 conducted various training exercises and participated in fleet maneuvers. It was decommissioned in 1931 and later sold for scrap in 1945. The submarine played a role in the early development of underwater warfare tactics for the U.S. Navy.