German submarine U-3503 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 17 June 1944 at F Schichau GmbH, Danzig, as yard number 1648. She was launched on 27 July 1944, and commissioned under the command of Oberleutnant zur See Hugo Deiring on 9 September 1944.

Like all Type XXI U-boats, U-3503 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-3503 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-3503 was scuttled on 8 May 1945, west of Gothenburg, Sweden, in the Kattegat, as part of Operation Regenbogen. The U-boat had earlier been sailing on the surface with U-534 and U-3523 just north of the cease-fire line when they had been attacked by allied B-24 Liberator planes. The wreck was raised in 1946 and broken up in Sweden (which first had asked to keep it) on demand of the Allied governments.

Her crew was taken up by HSwMS Norrköping and interned at Backamo internment camp.

The wreck was located at 57°39′N 11°44′E / 57.650°N 11.733°E / 57.650; 11.733.

The sea and marine museum Sjöfartsmuseet in Gothenburg still has the original Kriegsmarine flag from the U-boat, exhibited on one of the museum floors.

The closing track on Swedish musician Stefan Andersson's album Flygblad över Berlin is about the surrender of U-3503. The song is called "Den sista vargen" (The Last Wolf).

• Helgason, Guðmundur. "U-3503". German U-boats of WWII - uboat.net. Retrieved 22 April 2016.