U-66 may refer to one of the following German submarines:
, 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: , a Type UB III submarine launched in 1917 and went missing after 18 January 1918 , a Type UC II submarine launched in 1916 and sunk on 12 June 1917 , a Type IXC submarine that served in the Second World War until sunk on 6 May 1944