German submarine U-103 was a Type IXB U-boat of Nazi Germany's Kriegsmarine that operated during World War II.

She was one of the most successful boats in the entire war, sinking 238,944 gross register tons (GRT) of Allied shipping in 11 patrols, in a career lasting more than four years.

She was launched on 12 April 1940 and commissioned on 5 July under the command of Korvettenkapitän Victor Schütze.

After her warm-up, (designed to give her an opportunity to train and repair minor faults), she was deployed into the North Atlantic in September 1940 and saw overwhelming success, sinking 46 ships and damaging three other vessels.

The submarine was powered by two MAN M 9 V 40/46 supercharged four-stroke, nine-cylinder diesel engines producing a total of 4,400 metric horsepower (3,240 kW; 4,340 shp) for use while surfaced, two Siemens-Schuckert 2 GU 345/34 double-acting electric motors producing a total of 1,000 metric horsepower (740 kW; 990 shp) for use while submerged.

U-103's second patrol began with her being unsuccessfully attacked by the Flower-class corvette HMS Rhododendron northwest of Ireland on 11 November 1940.

The Flower-class corvette HMS Pimpernel rescued seven survivors, and landed them at Liverpool.

She sank Polyana 41 nmi (76 km; 47 mi) southwest of the Cape Verde Islands on 24 April 1941.

It sank Niceto de Larrinaga, Edward Blyden and Lapwing west of the Canary Islands on 22 September 1941.

The U-boat's sixth patrol was part of Operation Drumbeat, off the Atlantic coast of the USA.

She sank W. L. Stead about 90 nmi (170 km; 100 mi) off the Delaware River on 2 February 1942.

U-103 sank Tasmania north of Madeira on 31 October 1942, and Henry Stanley in mid-Atlantic northwest of the Azores on 6 December.

She searched the Atlantic off Spain as far west as the Azores and as far south as Morocco, but found no convoys.

The aircraft found the U-boat with its radar, and dropped six depth charges, but caused no damage.

U-103's last operational patrol was officially divided into two parts; the first of which saw her arrive in Brest in France only two days after her departure from Lorient.