German submarine U-865 was a Type IXC/40 U-boat of Nazi Germany's Kriegsmarine built for service during the Second World War. She was laid down in Bremen, Germany on 5 January 1943, and launched on 12 July 1943.
German Type IXC/40 submarines were slightly larger than the original Type IXCs. U-865 had a displacement of when at the surface and while submerged. The U-boat had a total length of , a pressure hull length of , a beam of , a height of , and a draught of . The submarine was powered by two MAN M 9 V 40/46 supercharged four-stroke, nine-cylinder diesel engines producing a total of for use while surfaced, two Siemens-Schuckert 2 GU 345/34 double-acting electric motors producing a total of for use while submerged. She had two shafts and two propellers. The boat was capable of operating at depths of up to .
The submarine had a maximum surface speed of and a maximum submerged speed of . When submerged, the boat could operate for at ; when surfaced, she could travel at . U-865 was fitted with six torpedo tubes (four fitted at the bow and two at the stern), 22 torpedoes, one SK C/32 naval gun, 180 rounds, and a Flak M42 as well as two twin C/30 anti-aircraft guns. The boat had a complement of forty-eight.
She had one commander, Oberleutnant zur See Dietrich Stellmacher, for her two patrols. She had a crew complement of 59.
She did not sink any shipping on her two patrols, and went missing after leaving Trondheim in Norway on 9 September 1944, with all hands lost.
In late June/ early July 1944, she was attacked by an RAF B-24 Liberator, which did some damage to her, however she in turn did some damage to the attacking aircraft with the submarine's flak gun, setting the aircraft on fire. Both the U-boat and aircraft were forced to return to their respective bases.