German submarine U-100 was a Type VIIB U-boat of Nazi Germany's Kriegsmarine during World War II.
German Type VIIB submarines were preceded by the shorter Type VIIA submarines. U-100 had a displacement of when at the surface and while submerged. She had a total length of , a pressure hull length of , a beam of , a height of , and a draught of . The submarine was powered by two Germaniawerft F46 four-stroke, six-cylinder supercharged diesel engines producing a total of for use while surfaced, two BBC GG UB 720/8 double-acting electric motors producing a total of for use while submerged. She had two shafts and two propellers. The boat was capable of operating at depths of up to .
The submarine had a maximum surface speed of and a maximum submerged speed of . When submerged, the boat could operate for at ; when surfaced, she could travel at . U-100 was fitted with five torpedo tubes (four fitted at the bow and one at the stern), fourteen torpedoes, one SK C/35 naval gun, 220 rounds, and one anti-aircraft gun. The boat had a complement of between 44 and 60.
The boat was launched on 10 April 1940, with a crew of 53, under the command of Kapitänleutnant Joachim Schepke. On her first active patrol, U-100 came into contact with two Allied convoys, OA-198 and OA-204. She shadowed both convoys.
U-100 departed for her second active patrol on 11 September 1940, coming into contact with the Allied convoy HX 72. HX 72 lost 11 ships in total, with U-100 accounting for 7 ships of 50,340 GRT. In the attack on this convoy, while other U-boats stood off to the side and fired their torpedoes to little or no success, U-100 penetrated inside the convoy before attacking, a tactic soon adopted by the C-in-C of U-boats, Admiral Karl Donitz.
After resupplying, U-100 departed for her third active patrol on 12 October 1940. She came into contact with two Allied convoys, HX 79 and SC 7.
U-100 departed on her fourth patrol on 7 November 1940. On 22 November she came into contact with the Allied convoy SC 11 and began to shadow it.
U-100 left for her fifth active patrol on 2 December 1940, sinking two vessels from Convoy OB 256, then a third solo vessel.
U-100 departed on her sixth and what would be her final patrol on 9 March 1941. She approached convoy HX 112 from astern in the pre-dawn hours of 17 March, but was detected at a range of 1,000 meters by the Type 286 radar aboard . U-100 was the first U-boat to be so discovered during World War II; she was rammed and sunk by Vanoc while attempting to submerge. Another destroyer, , was also present. Six of the boat's 53 crew members survived, spending the remainder of the war as POWs. Schepke was not one of them.