German submarine U-40 may refer to one of the following German submarines:

• SM U-40 (Germany), was a Type U 31 submarine launched in 1914 and that served in the First World War until sunk 23 June 1915
  • During the First World War, Germany also had these submarines with similar names:
    • SM UB-40, a Type UB II submarine launched in 1916 and scuttled on 5 October 1918
    • SM UC-40, a Type UC II submarine launched in 1916 and sunk on 21 January 1919 while on way to surrender
• German submarine U-40 (1938), a Type IX submarine that served in the Second World War until sunk on 13 October 1939