German submarine U-55 was a Type VIIB U-boat of Nazi Germany's Kriegsmarine during World War II. She was ordered on 16 July 1937 and laid down on 2 November 1938 at Friedrich Krupp Germaniawerft in Kiel as yard number 590. Launched on 19 October 1939, she went into service on 21 November 1939 under the command of Kapitänleutnant (Kptlt.) Werner Heidel.
German Type VIIB submarines were preceded by the shorter Type VIIA submarines. U-55 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 MAN M 6 V 40/46 four-stroke, six-cylinder supercharged diesel engines producing a total of for use while surfaced, two AEG GU 460/8-276 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-55 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 forty-four and sixty.
U-55 began her first and only war patrol on 16 January 1940, under Heidel's command; he had previously sunk two ships in . She sank four small freighters sailing independently, then attacked convoy OA-80G on 29 January. U-55 sank two more ships before coming under concerted attack from the convoy's escorts, supported by a Sunderland flying boat from RAF Coastal Command 228 Squadron.
After a sustained depth charge attack, the U-boat surfaced and carried out a running gun battle before her deck gun jammed. Heidel ordered the boat to be abandoned, then apparently went down with it. The remainder of the crew was rescued by the escorts. The British awarded official credit for sinking U-55 to the sloop , the destroyer , French destroyers Valmy and Guépard, and the Sunderland.