- Class: U19 - 4 submarines of 1912
- Displacement: 650/837 tons
- Dimensions: 210.5 x 20 x 11.75 ft.
- Machinery: Diesel engines, 2 screws. 1,700/1,200 h.p. = 15.4/9.5 knots
- Armament: 4-19.7 in. T.T. (2 bow, 2 stern); 1-3.4 in. gun
- Complement: 35
- Builder: Kaiserliche Werft, Danzig, Germany
- Launched: 8 Feb 1913
- Completed: 1913
- Actions: Commissioned one year before the start of World War I under the command of LtCmdr Otto Hersing. She was the first submarine to sink a warship on the high seas. Torpedoed the British light cruiser HMS Pathfinder on 5 Sep 1914 in the North Sea off the Firth of Forth, which sank in 11 minutes with all but 3 out of her crew of 350. Sank a number of merchant ships under prize law, including three in the Irish sea off Liverpool on 30 Jan 1915. Departed from Wilhelmshaven 25 Apr 1915 for the Mediterranean, passing Gibraltar on 6 May and arriving at Cattaro in the Adriatic on 13 May with only 1.8 tons of fuel left. A week later on May 25 she sank the battleship HMS Triumph and two days later, the battleship HMS Majestic off Cape Helles, forcing allied capital ships away from the Dardanelles. U21 sailed into Istanbul on 5 Jun. The Kaiser awarded her crew the Iron Cross First Class and the Pour le Merite to Hersing. She sank the French transport Carthage on 4 Jul, but was latered damaged by a probable mine and put out of action for an extended period. On 8 Feb 1916 she sank the French cruiser Amiral Charner, with the loss of all but 1 of a crew of 335. In Apr 1916 she sank the British merchantman City of Lucknow about 60 miles east of Malta. Recalled to Germany in 1917, she resumed patrols around Britain and then was used for training. Ordered to surrender, instead Hersing scuttled her 22 Feb 1919 in the North Sea.