U-138 may refer to one of the following German submarines:
- SM U-138, a Type U 127 submarine launched in 1917 and that served in the First World War
- During the First World War, Germany also had this submarine with a similar name:
- SM UB-138, a Type UB III submarine laid down but unfinished at the end of the war; broken up on the slip in 1919
- During the First World War, Germany also had this submarine with a similar name:
- German submarine U-138 (1940), a Type IID submarine that served in the Second World War until sunk on 18 June 1941
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.