German submarine U-866 was a Type IXC/40 U-boat of the Nazi German Kriegsmarine built for service during the Second World War. She was ordered on 25 August 1941, laid down in Bremen, Germany on 23 January 1943 and launched on 29 July 1943. She had three patrols, her first training patrol in 1943, and then two patrols in 1944. She had a complement of 54. Her commander from 17 November 1943 to December 1944 was Korvettenkapitän Walter Pommerehne, followed by Oberleutnant zur See Peter Rogowsky, who commanded her from December 1944 to 18 March 1945.