German submarine U-523 was a Type IXC U-boat of Nazi Germany's Kriegsmarine during World War II.

The submarine was powered by two MAN M 9 V 40/46 supercharged four-stroke, nine-cylinder diesel engines producing a total of 4,400 metric horsepower (3,240 kW; 4,340 shp) for use while surfaced, two Siemens-Schuckert 2 GU 345/34 double-acting electric motors producing a total of 1,000 shaft horsepower (1,010 PS; 750 kW) for use while submerged.

[5] 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 U-527.

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.

[6] The U-boat arrived at her new home port of Lorient, in occupied France, on 16 April 1943 after 67 days at sea.