|Ordered||25 August 1941|
|Builder||H. C. Stülcken Sohn, Hamburg|
|Laid down||16 November 1942|
|Launched||23 July 1943|
|Commissioned||8 November 1943|
|Fate||scuttled in May 1945|
|Class and type||Type VIIC submarine|
|Height||9.60 m (31 ft 6 in)|
|Draught||4.74 m (15 ft 7 in)|
|Complement||4 officers, 40–56 enlisted|
|Identification codes:||M 53 697|
|Commanders:||Oblt.z.S. Otto Wollschläger|
|Operations:||no war patrols|
|Victories:||no ships sunk|
German submarine U-721 was a Type VIIC U-boat of Nazi Germany's Kriegsmarine during World War II. The submarine was laid down on 16 November 1942 at the H. C. Stülcken Sohn yard at Hamburg, launched on 23 July 1943, and commissioned on 8 November 1943 under the command of Oberleutnant zur See Otto Wollschläger.
German Type VIIC submarines were preceded by the shorter Type VIIB submarines. U-721 had a displacement of 769 tonnes (757 long tons) when at the surface and 871 tonnes (857 long tons) while submerged. She had a total length of 67.10 m (220 ft 2 in), a pressure hull length of 50.50 m (165 ft 8 in), a beam of 6.20 m (20 ft 4 in), a height of 9.60 m (31 ft 6 in), and a draught of 4.74 m (15 ft 7 in). The submarine was powered by two Germaniawerft F46 four-stroke, six-cylinder supercharged diesel engines producing a total of 2,800 to 3,200 metric horsepower (2,060 to 2,350 kW; 2,760 to 3,160 shp) for use while surfaced, two Garbe, Lahmeyer & Co. RP 137/c double-acting electric motors producing a total of 750 metric horsepower (550 kW; 740 shp) for use while submerged. She had two shafts and two 1.23 m (4 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 17.7 knots (32.8 km/h; 20.4 mph) and a maximum submerged speed of 7.6 knots (14.1 km/h; 8.7 mph). When submerged, the boat could operate for 80 nautical miles (150 km; 92 mi) at 4 knots (7.4 km/h; 4.6 mph); when surfaced, she could travel 8,500 nautical miles (15,700 km; 9,800 mi) at 10 knots (19 km/h; 12 mph). U-721 was fitted with five 53.3 cm (21 in) torpedo tubes (four fitted at the bow and one at the stern), fourteen torpedoes, one 8.8 cm (3.46 in) SK C/35 naval gun, 220 rounds, and two twin 2 cm (0.79 in) C/30 anti-aircraft guns. The boat had a complement of between forty-four and sixty.
The final action of the submarine was during Operation Hannibal when it evacuated some 100 civilians and wounded soldiers. It was scuttled on 4 May 1945 in Geltinger Bucht as part of Operation Regenbogen (U-boat) and later raised and broken up for scrap.