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.

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.

[4] The closing track on Swedish musician Stefan Andersson's album Flygblad över Berlin is about the surrender of U-3503.