U-23 may refer to one of the following German submarines:
- SM U-23 (Germany), was the lead ship of the Type U 23 class of submarines; launched in 1913 and served in the First World War until sunk on 20 July 1915
- During the First World War, Germany also had these submarines with similar names:
- SM UB-23, a Type UB II submarine launched in 1915 and interned in Spain 29 July 1917
- SM UC-23, a Type UC II submarine launched in 1916 and surrendered on 25 November 1918
- During the First World War, Germany also had these submarines with similar names:
- German submarine U-23 (1936), a Type IIB submarine that served in the Second World War until scuttled on 10 September 1944
- German submarine U-23 (S172), a Type 206 submarine of the Bundesmarine that was launched in 1974 and sold after decommissioning to Columbia where she serves as ARC Intrépido.
U-23 or U-XXIII may also refer to:
- SM U-23 (Austria-Hungary), a U-20-class submarine of the Austro-Hungarian Navy
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.