U-78 may refer to one of the following German submarines:
- SM U-78, a Type UE I submarine launched in 1916 and that served in World War I until sunk 27 October 1917
- During World War I, Germany also had these submarines with similar names:
- SM UB-78, a Type UB III submarine launched in 1917 and sunk on 19 April 1918
- SM UC-78, a Type UC II submarine launched in 1916 and sunk on 9 May 1918
- During World War I, Germany also had these submarines with similar names:
- German submarine U-78, a Type VIIC submarine that served in World War II until sunk on 16 April 1945
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.