German submarine U-864 was a Type IX U-boat of Nazi Germany's Kriegsmarine in World War II. She departed from Kiel on 5 December 1944 on her last mission, to transport to Japan a large quantity of mercury and parts and engineering drawings for German jet fighters. While returning to Bergen, Norway to repair a misfiring engine, U-864 was detected and sunk on 9 February 1945 by the British submarine , killing all 73 on board. It is the only instance in the history of naval warfare where one submarine intentionally sank another while both were submerged.