The STEN (or Sten gun) is a family of British submachine guns chambered in 9×19mm which were used extensively by British and Commonwealth forces throughout World War II and the Korean War. They had a simple design and very low production cost, making them effective insurgency weapons for resistance groups, and they continue to see usage to this day by irregular military forces. The Sten served as the basis for the Sterling submachine gun, which replaced the Sten in British service until the 1990s, when it, and all other submachine guns, were replaced by the SA80.

The Sten is a select fire, blowback-operated weapon which mounts its magazine on the left. Sten is an acronym, from the names of the weapon's chief designers, Major Reginald V. Shepherd and Harold J. Turpin, and "En" for the Enfield factory. Over four million Stens in various versions were made in the 1940s, making it the second most produced submachine gun of the Second World War, after the Soviet PPSh-41.