Unterseeboot 9 has been the designation of two U-boats.

World War I

During World War I, U-9 was the lead ship of her class of U-boat of the Kaiserliche Marine. Her construction was ordered on 15 July 1908 and her keel was laid down by Kaiserliche Werft in Danzig. She was launched on 22 February 1910 and she was commissioned on 18 April 1910.

On 16 July 1914 the crew of U-9 reloaded her torpedo tubes while submerged, the first time any submarine had succeeded in doing so.

On 1 August 1914, Leutnant zur See Otto Weddigen took command. On 22 September 1914, while patrolling in the region of the southern North Sea known to the British as the "Broad Fourteens," U-9 found the "Live Bait Squadron ." She shot all six of her torpedoes, reloading while submerged, and in less than an hour sank the three British armored cruisers HMS Aboukir , Hogue , and Cressy . Almost 1400 British sailors died. Members of the Admiralty who had considered submarines to be mere "toys" no longer expressed that opinion after this event.

On 12 January 1915, Johannes Spieß relieved Weddigen, and commanded U-9 until 19 April 1916.

U-9 was surrendered to the British on 26 November 1918, and was broken up at Morecambre in 1919.

In total, U-9 conducted seven patrols, sinking 13 merchant ships totalling 8636 tons.

World War II

During World War II, U-9 was a Type IIB submarine of the Kriegsmarine. Her keel was laid down on 8 April 1935 by Germaniawerft in Kiel. She was launched on 30 July 1935 and commissioned on 21 August 1935 with Kapitänleutnant Hans-Günther Looff in command.

U-9 conducted 19 patrols under a series of commanders, sinking eight ships totalling 17,221 tons and damaging another displacing 412 tons. This included the French coastal submarine of the Sirène class Doris.

At 1030 on 20 August 1944 at Konstanza in the Black Sea, U-9 was sunk by bombs from Soviet aircraft. In 1945 the Soviets raised the boat and brought it into Nikolaev. She was repaired and in 1945 was commissioned in to the Soviet Navy as Soviet submarine TS-16 , but did not perform well and was broken up after 12 December, 1946.