German submarine U-21

U-21 may refer to one of the following German submarines:


 * SMU U-21 (Germany), was a Type U 19 submarine launched in 1913 and that served in the First World War; sank HMS Pathfinder (1904), the first ship sunk by a torpedo fired from a submarine; U-21 sank on 22 February 1919 while en route to be surrendered
 * During the First World War, Germany also had these submarines with similar names:
 * SMU UB-21, a Type UB II submarine launched in 1915 and surrendered 24 November 1918
 * SMU UC-21, a Type UC II submarine launched in 1916 and lost in September 1917
 * GS U-21 (1936), a Type IIB submarine that served in the Second World War; stricken on 5 August 1944 and scrapped in February 1945
 * GS U-21 (S170), a Type 206 submarine of the Bundesmarine that was launched in 1974 and scrapped in 1998

U-21 or U-XXI may also refer to:
 * SMU U-21 (Austria-Hungary), a U-20 class submarine of the Austro-Hungarian Navy