[1] At 04:45 on 4 October 1939, U-23 scored one of the Kriegsmarine's early successes of the war when she torpedoed and sank with gunfire, the merchant ship Glen Farg about 60 nmi (110 km; 69 mi) south-southwest of Sumburgh Head (southern Shetland).

Over the course of her service with the Kriegsmarine, U-23 had ten commanding officers, the most famous of whom was Kapitänleutnant Otto Kretschmer, who went on to become the top scoring U-boat ace.

She was then re-assembled by the Romanians at the Galați shipyard and sent to the Black Sea port of Constanţa, Romania, with the 30th U-boat Flotilla until September 1944.

[2] U-23 was scuttled by her crew on 10 September 1944, off the coast of Turkey in the Black Sea at position 41°11′N 30°00′E / 41.183°N 30.000°E / 41.183; 30.000 to prevent her capture by the advancing Soviets.

[citation needed] On 3 February 2008, The Daily Telegraph newspaper reported that U-23 had been discovered by Selçuk Kolay, a Turkish marine engineer, in 160 ft (49 m) of water, three miles from the town of Ağva.