German submarine U-523 was a Type IXC U-boat of Nazi Germany's Kriegsmarine during World War II. The submarine was laid down on 4 August 1941 at the Deutsche Werft yard in Hamburg as yard number 338. She was launched on 15 April 1942, and commissioned on 25 June under the command of Kapitänleutnant Werner Pietzsch. After training with the 4th U-boat Flotilla in the Baltic Sea, the U-boat was transferred to the 10th flotilla for front-line service on 1 February 1943.
German Type IXC submarines were slightly larger than the original Type IXBs. U-523 had a displacement of when at the surface and while submerged. The U-boat 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 9 V 40/46 supercharged four-stroke, nine-cylinder diesel engines producing a total of for use while surfaced, two Siemens-Schuckert 2 GU 345/34 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-523 was fitted with six torpedo tubes (four fitted at the bow and two at the stern), 22 torpedoes, one SK C/32 naval gun, 180 rounds, and a SK C/30 as well as a C/30 anti-aircraft gun. The boat had a complement of forty-eight.
During her training period in the Baltic Sea with the 4th U-boat Flotilla, U-523 executed tests with the Focke-Achgelis Fa 330 rotor kite.
U-523 departed Kiel on 9 February 1943 and sailed out into the mid-Atlantic. On the morning of 19 March the 5,848 GRT American merchant ship Mathew Luckenbach, part of Convoy HX 229 en route to the UK from New York City, was hit by two torpedoes fired by . Around 20:00 that evening, U-523 discovered the drifting wreck of the Mathew Luckenbach and hit her with a single torpedo, sinking the ship within seven minutes. The U-boat arrived at her new home port of Lorient, in occupied France, on 16 April 1943 after 67 days at sea.
U-523 sailed from Lorient on 22 May 1943, but on the 24th, still in the Bay of Biscay, she was bombed by a British Whitley medium bomber of No. 10 Operational Training Unit RAF. The U-boat was severely damaged and was forced to return to Lorient.
U-523 sailed from Lorient briefly on 1 August 1943, for a voyage lasting only three days, before setting out once more on 16 August, and headed south-west.
The U-boat was sunk on 25 August, west of Vigo, Spain, in position , by depth charges from the destroyer and the corvette . Seventeen of U-523s crew were killed and 37 survived the attack.
U-523 took part in four wolfpacks, namely: