German submarine U-20

U-20 may refer to one of the following German submarines:


 * SMU U-20 (Germany), was a Type U 19 submarine launched in 1912 and that served in the First World War until grounded on 4 November 1916; notable for sinking RMS Lusitania
 * During the First World War, Germany also had these submarines with similar names:
 * SMU UB-20, a Type UB II submarine launched in 1915 and sunk 28 July 1917
 * SMU UC-20, a Type UC II submarine launched in 1916 and surrendered on 16 January 1919
 * GS U-20 (1936), a Type IIB submarine that served in the Second World War and was scuttled on 10 September 1944
 * GS U-20 (S199), a Type 206 submarine of the Bundesmarine that was launched in 1974 and scrapped in 1996 and sold to Indonesia

U-20 or U-XX may also refer to:
 * SMU U-20 (Austria-Hungary), lead boat of the U-20 class submarines for the Austro-Hungarian Navy