U-84 may refer to one of the following German submarines:
- SM U-84, a Type U 81 submarine launched in 1916 and that served in the First World War until sunk on 26 January 1918
- During the First World War, Germany also had this submarine with a similar name:
- SM UB-84, a Type UB III submarine launched in 1917 and surrendered on 26 November 1918; broken up at Brest in 1921
- During the First World War, Germany also had this submarine with a similar name:
- German submarine U-84 (1941), a Type VIIB submarine that served in the Second World War until sunk on 7 August 1943
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.