U-121 may refer to one of the following German submarines:
- SM U-121, a Type UE II submarine launched in 1918 and that served in World War I; surrendered 1918; sunk as French gunnery target on 1 July 1921
- During World War I, Germany also had this submarine with a similar name:
- SM UB-121, a Type UB III submarine launched in 1918 and surrendered in 1918; wrecked in tow to France 1919
- During World War I, Germany also had this submarine with a similar name:
- German submarine U-121 (1940), a Type IIB submarine that served in World War II and was scuttled on 2 May 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.