German submarine U-3523 was a Type XXI U-boat (one of the "Elektroboote") of Nazi Germany's Kriegsmarine, built for service in World War II. She was ordered on 6 November 1943, and was laid down on 7 October 1944 at F Schichau GmbH, Danzig, as yard number 1668. She was launched on 14 December 1944, and commissioned under the command of Oberleutnant zur See Willi Müller on 23 January 1945.

Like all Type XXI U-boats, U-3523 had a displacement of 1,621 tonnes (1,595 long tons) when at the surface and 1,819 tonnes (1,790 long tons) while submerged. She had a total length of 76.70 m (251 ft 8 in) (o/a), a beam of 8 m (26 ft 3 in), and a draught of 6.32 m (20 ft 9 in). The submarine was powered by two MAN SE supercharged six-cylinder M6V40/46KBB diesel engines each providing 4,000 metric horsepower (2,900 kilowatts; 3,900 shaft horsepower), two Siemens-Schuckert GU365/30 double-acting electric motors each providing 5,000 PS (3,700 kW; 4,900 shp), and two Siemens-Schuckert silent running GV232/28 electric motors each providing 226 PS (166 kW; 223 shp).

The submarine had a maximum surface speed of 15.6 knots (28.9 km/h; 18.0 mph) and a submerged speed of 17.2 knots (31.9 km/h; 19.8 mph). When running on silent motors the boat could operate at a speed of 6.1 knots (11.3 km/h; 7.0 mph). When submerged, the boat could operate at 5 knots (9.3 km/h; 5.8 mph) for 340 nautical miles (630 km; 390 mi); when surfaced, she could travel 15,500 nautical miles (28,700 km; 17,800 mi) at 10 knots (19 km/h; 12 mph). U-3523 was fitted with six 53.3 cm (21.0 in) torpedo tubes in the bow and four 2 cm (0.79 in) C/30 anti-aircraft guns. She could carry twenty-three torpedoes, or seventeen torpedoes and twelve mines. The complement was five officers and fifty-two men.

U-3523 had been sailing on the surface with U-534 and U-3503 just north of the cease-fire line when it was sunk by depth charges from a British B-24 Liberator of 86 Squadron/G RAF about 10 nautical miles (19 km; 12 mi) north of Skagen Horn, in the Skagerrak on 6 May 1945. All 58 crewmen were lost.

The wreck was thought to be located at 57°52′N 10°49′E / 57.867°N 10.817°E / 57.867; 10.817, however, in April 2018, it was found by Sea War Museum Jutland, Thyborøn, 9 nautical miles (17 km; 10 mi) further west. The wreck lies at a depth of 123 metres (404 ft). The entire fore part of the boat is buried in the seabed while its stern rises 20 metres (66 ft) above the bottom.

• JD-Contractor A/S. "Sea War Museum Jutland finds new WWII submarine!". YouTube. Retrieved 13 April 2018.

• Helgason, Guðmundur. "U-3523". German U-boats of WWII - uboat.net. Retrieved 26 April 2016.