German submarine U-851 was a long-range Type IXD2 U-boat built for Nazi Germany's Kriegsmarine during World War II.
She was ordered on 20 January 1941, and was laid down on 18 March 1942 at DeSchiMAG AG Weser, Bremen, as yard number 1057. She was launched on 15 January 1943 and commissioned under the command of Korvettenkapitän Hannes Weingärtner on 21 May 1943.
German Type IXD2 submarines were considerably larger than the original Type IXs. U-851 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 plus two MWM RS34.5S six-cylinder four-stroke diesel engines for cruising, 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-851 was fitted with six torpedo tubes (four fitted at the bow and two at the stern), 24 torpedoes, one SK C/32 naval gun, 150 rounds, and a SK C/30 with 2575 rounds as well as two C/30 anti-aircraft guns with 8100 rounds. The boat had a complement of fifty-five.
U-851 was last heard from on 27 March 1944, thirty-one days into her first, and only, war patrol. She is presumed sunk in the North Atlantic with all 70 of her crew missing.
Her last known position was southeast of St. John's, Newfoundland .