German submarine U-51 was a Type VIIB U-boat of the Nazi German Kriegsmarine that operated during World War II. She was ordered in November 1936 and laid down in February 1937 in Kiel. She was launched in August 1939 and commissioned in November. During her service in the Kriegsmarine, U-51 conducted four war patrols and sank five enemy vessels for a loss of and one auxiliary warship of 4,724 GRT. She was a member of one wolf pack. She was sunk in August 1940 in the Bay of Biscay by a torpedo from a British submarine.