U-31 may refer to one of the following German submarines:
- SM U-31 (Germany), was the lead ship of the Type U 31 class of submarines; launched in 1914 and served in the First World War until sunk on 13 January 1915
- During the First World War, Germany also had these submarines with similar names:
- SM UB-31, a Type UB II submarine launched in 1915 and sunk on 2 May 1918
- SM UC-31, a Type UC II submarine launched in 1916 and surrendered on 26 November 1918
- During the First World War, Germany also had these submarines with similar names:
- German submarine U-31 (1936), a Type VIIA submarine that served in the Second World War until sunk on 2 November 1940
- German submarine U-31 (S181), a Type 212 submarine of the Bundesmarine that was launched in 2002 and still in service
This article includes a list of ships with the same or similar names. If an internal link for a specific ship led you here, you may wish to change the link to point directly to the intended ship article, if one exists.