German submarine U-521 was a Type IXC U-boat of the Nazi German Kriegsmarine during World War II. She was built by the Deutsche Werft yard at Finkenwerder, Hamburg with yardnumber 336. Commissioned in June 1942, she was commanded by Kapitänleutnant Klaus Bargsten. The U-boat was assigned to the 4th U-boat Flotilla for training and the 2nd flotilla for operations.