German submarine U-503 was a Type IXC U-boat of Nazi Germany's Kriegsmarine during World War II. The submarine was laid down on 29 April 1940 at the Deutsche Werft yard in Hamburg as yard number 293, launched on 5 April 1941 and commissioned on 10 July 1941 under the command of Kapitänleutnant Otto Gericke.
The boat's service began with her being assigned to the 2nd U-boat Flotilla on her commissioning date for training and continuing with it for operations from 1 February 1942.
German Type IXC submarines were slightly larger than the original Type IXBs. U-503 had a displacement of 1120t when at the surface and 1232t while submerged. The U-boat had a total length of 76.76m (251.84feet), a pressure hull length of 58.75m (192.75feet), a beam of 6.76m (22.18feet), a height of 9.6m (31.5feet), and a draught of 4.7m (15.4feet). The submarine was powered by two MAN M 9 V 40/46 supercharged four-stroke, nine-cylinder diesel engines producing a total of 4400PS for use while surfaced, two Siemens-Schuckert 2 GU 345/34 double-acting electric motors producing a total of 1000shp for use while submerged. She had two shafts and two 1.92abbr=onNaNabbr=on 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-503 was fitted with six 53.30NaN0 torpedo tubes (four fitted at the bow and two at the stern), 22 torpedoes, one 10.52NaN2 SK C/32 naval gun, 180 rounds, and a 3.7sp=usNaNsp=us SK C/30 as well as a 2sp=usNaNsp=us C/30 anti-aircraft gun. The boat had a complement of forty-eight.
U-503s first and only active war patrol began at Bergen in Norway on 28 February 1942, following two previous port to port sailings. The U-boat was sunk on 15 March by depth charges dropped by a PBO-1 Hudson of United States Navy squadron VP-82 south-east of Newfoundland, in position 45.8333°N -98°W.
The aircraft, from Argentia, was escorting Convoy ON-72. The PBO-1s were twenty Hudson Mk.IIIA aircraft diverted from Lend-Lease to equip VP-82, and sank the first two U-boats by US forces; on 1 March 1942 and U-503.