U-21 may refer to one of the following German submarines:
- SM U-21 (Germany), was a Type U 19 submarine launched in 1913 and that served in the First World War; sank HMS Pathfinder, the first ship sunk by a torpedo fired from a submarine; U-21 sank on 22 February 1919 while en route to be surrendered
- German submarine U-21 (1936), a Type IIB submarine that served in the Second World War; stricken on 5 August 1944 and scrapped in February 1945
- German submarine 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:
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