German submarine U-66

U-66 may refer to one of the following German submarines:


 * SMU U-66, the lead ship of the Type U 66 class of submarines; launched in 1915 and that served in the First World War until sunk on 3 September 1917
 * During the First World War, Germany also had these submarines with similar names:
 * SMU UB-66, a Type UB III submarine launched in 1917 and went missing after 18 January 1918
 * SMU UC-66, a Type UC II submarine launched in 1916 and sunk on 12 June 1917
 * GS U-66 (1940), a Type IXC submarine that served in the Second World War until sunk on 6 May 1944