German submarine U-104 was a Type IXB U-boat of the Nazi German Kriegsmarine during World War II. She was ordered by the Kriegsmarine on 24 May 1938 as part of the German naval rearmament program Plan Z. Her keel was laid down by AG Weser in Bremen in November 1939. Following about six and a half months of construction, she was launched in May 1940 and formally commissioned into the Kriegsmarine in August 1940.