U-23 may refer to one of the following German submarines:
, 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: , a Type UB II submarine launched in 1915 and interned in Spain 29 July 1917 , a Type UC II submarine launched in 1916 and surrendered on 25 November 1918 , a Type IIB submarine that served in the Second World War until scuttled on 10 September 1944 , a Type 206 submarine of the Bundesmarine that was launched in 1974 and sold after decommissioning to Columbia where she serves as .
U-23 or U-XXIII may also refer to: , a submarine of the Austro-Hungarian Navy