|Ordered||25 August 1941|
|Builder||DeSchiMAG AG Weser, Bremen|
|Laid down||23 January 1943|
|Launched||29 July 1943|
|Commissioned||17 November 1943|
|Fate||Sunk on 18 March 1945|
|Class and type||Type IXC/40 submarine|
|Height||9.60 m (31 ft 6 in)|
|Draught||4.67 m (15 ft 4 in)|
|Test depth||230 m (750 ft)|
|Complement||4 officers, 44 enlisted|
German submarine U-866 was a Type IXC/40 U-boat of Nazi Germany's 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.
German Type IXC/40 submarines were slightly larger than the original Type IXCs. U-866 had a displacement of 1,144 tonnes (1,126 long tons) when at the surface and 1,257 tonnes (1,237 long tons) while submerged. The U-boat had a total length of 76.76 m (251 ft 10 in), a pressure hull length of 58.75 m (192 ft 9 in), a beam of 6.86 m (22 ft 6 in), a height of 9.60 m (31 ft 6 in), and a draught of 4.67 m (15 ft 4 in). The submarine was powered by two MAN M 9 V 40/46 supercharged four-stroke, nine-cylinder diesel engines producing a total of 4,400 metric horsepower (3,240 kW; 4,340 shp) for use while surfaced, two Siemens-Schuckert 2 GU 345/34 double-acting electric motors producing a total of 1,000 shaft horsepower (1,010 PS; 750 kW) for use while submerged. She had two shafts and two 1.92 m (6 ft) propellers. The boat was capable of operating at depths of up to 230 metres (750 ft).
The submarine had a maximum surface speed of 18.3 knots (33.9 km/h; 21.1 mph) and a maximum submerged speed of 7.3 knots (13.5 km/h; 8.4 mph). When submerged, the boat could operate for 63 nautical miles (117 km; 72 mi) at 4 knots (7.4 km/h; 4.6 mph); when surfaced, she could travel 13,850 nautical miles (25,650 km; 15,940 mi) at 10 knots (19 km/h; 12 mph). U-866 was fitted with six 53.3 cm (21 in) torpedo tubes (four fitted at the bow and two at the stern), 22 torpedoes, one 10.5 cm (4.13 in) SK C/32 naval gun, 180 rounds, and a 3.7 cm (1.5 in) Flak M42 as well as two twin 2 cm (0.79 in) C/30 anti-aircraft guns. The boat had a complement of forty-eight.
While under command of Rogowsky, on 16 March, USS Lowe acquired U-866 on sonar and commenced a hedgehog attack. This attack missed the U-boat, which then settled on the ocean floor, attempting to hide from the attacking surface ships. Unfortunately for the U-boat, the seabed in the area was ideal for the surface ship's sonar and USS Lowe, USS Menges, USS Mosley, and USS Pride, all destroyer escorts, continued to attack with depth charges, until the U-boat was judged destroyed.
She did not sink or damage any ships while on patrol.