German submarine U-42 was a Type IXA U-boat of the Nazi German Kriegsmarine that operated during World War II. U-42 was ordered by the Kriegsmarine in November 1936. Her keel was laid down in December 1937; she was launched in February 1939 and commissioned in July 1939. U-42 had a very short career, being sunk while still on her first war patrol. During her service with the Kriegsmarine, the boat conducted only one training patrol and one war patrol. Over the latter she damaged one enemy vessel of . Both her patrols were as part of the 6th U-boat Flotilla.