The first U-2, the lead ship of her type, was ordered from Kaiserliche Werft of Danzig on 4 March 1906. She was launched on 18 June 1908, and commissioned into the Kaiserliche Marine on 18 July 1908. She conducted no war patrols and spent World War I as a training platform. After Germany's surrender, she was broken up at Stinnes on 19 February 1919.
The second U-2 was a Type IIA submarine of the Kriegsmarine. Her keel was laid down 11 February 1935 by Deutsche Werke of Kiel, and she was commissioned 25 July 1935 with Oberleutnant zur See Hermann Michahelles in command.
She had several commanders over her long career. Michahelles was relieved on 30 September 1936, by Käpitanleutant Heinrich Liebe. Liebe turned command over on 31 January 1938, to Oblt. Herbert Schultze. On 16 March 1939, Kptlt. Helmut Rosenbaum assumed command and on 7 July 1940, Oblt. Hans Heidtmann joined Rosenbaum as deputy commander. On 6 August 1940, Georg von Wilamowitz-Moellendorf relieved Rosenbaum and Heidtmann and commanded until October of 1941 when Karl Kölzer took over. On 16 May 1942, Oblt. Werner Schwaff relieved Kölzer, and on 20 November 1942, was relieved by Oblt. Helmut Herglotz. On 12 December 1943, Oblt. Wolfgang Schwarzkopf took over and commanded the boat until she was lost.
She was used as a school boat and trainer for her entire career except for two completely uneventful combat patrols in the spring of 1940.
U-2 suffered no casualties to any of her numerous crews until 8 April 1944 when she collided with the German steam trawler Helmi Söhle west of Pillau and sank. Seventeen of her crew died; 18 survived. The wreck was raised the next day and stricken.