German submarine U-51 was a Type VIIB U-boat of Nazi Germany's Kriegsmarine that operated during World War II. She was ordered on 21 November 1936 and laid down on 26 February 1937 in Kiel. She was launched on 11 June 1938 and commissioned on 6 August 1938. During her service in the Kriegsmarine, U-51 conducted four war patrols and sank five enemy vessels for a loss of 26,296 gross register tons (GRT) and one auxiliary warship of 4,724 GRT. She was a member of one wolfpack. She was sunk on 20 August 1940 in the Bay of Biscay by a torpedo from a British submarine.