U-81 may refer to one of the following German submarines:
- SM U-81, a Type U 81 submarine launched in 1916 and that served in the First World War until sunk on 1 May 1917
- During the First World War, Germany also had this submarine with a similar name:
- SM UB-81, a Type UB III submarine launched in 1917 and sunk on 2 December 1917
- During the First World War, Germany also had this submarine with a similar name:
- German submarine U-81 (1941), a Type VIIC submarine that served in the Second World War until sunk on 9 January 1944; raised on 22 April 1944 and broken up
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.