German submarine U-678 was a Type VIIC U-boat built for Nazi Germany's Kriegsmarine for service during World War II. She was laid down on 3 September 1942 by Howaldtswerke, Hamburg as yard number 827, launched on 18 September 1943 and commissioned on 25 October 1943 under Oberleutnant zur See Guido Hyronimus.
German Type VIIC submarines were preceded by the shorter Type VIIB submarines. U-678 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 Siemens-Schuckert GU 343/38-8 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-678 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), one Flak M42 and two twin C/30 anti-aircraft guns. The boat had a complement of between forty-four and sixty.
The boat's career began with training at 5th U-boat Flotilla on 25 October 1943, followed by active service on 1 June 1944 as part of the 7th Flotilla for the remainder of her short service. In one patrol she sank no ships. U-678 was sunk on 6 July 1944 in the English Channel in position , by depth charges from the Canadian destroyers , and the Royal Navy corvette . All crew members died.