German submarine U-91 was a Type VIIC U-boat of Nazi Germany's Kriegsmarine during World War II.

She was laid down at the Flender Werke in Lübeck as yard number 295, launched on 30 November 1941 and commissioned on 28 January 1942 with Oberleutnant zur See Heinz Walkerling as commanding officer.

She was a fairly successful boat, sinking over 26,000 tons of Allied shipping in a career lasting just 14 months and six patrols.

The submarine was powered by two Germaniawerft F46 four-stroke, six-cylinder supercharged diesel engines producing a total of 2,800 to 3,200 metric horsepower (2,060 to 2,350 kW; 2,760 to 3,160 shp) for use while surfaced, two Brown, Boveri & Cie GG UB 720/8 double-acting electric motors producing a total of 750 metric horsepower (550 kW; 740 shp) for use while submerged.

At 02:15, the submarine came across the damaged Ottawa once again, but mistook her for a different vessel and fired a third torpedo, which destroyed the ship, killing 114 of the 181 men aboard.

U-91 was subject to a "rain of aircraft bombs and depth charges from surface ships" which obliged her to break off an attack to carry out repairs on 21 February 1943.

Two American vessels - SS Harry Luckenbach and Irénée Du Pont - were destroyed along with the British merchant ship Nariva.

Luckenbach sank in a mere three minutes, with seventy-one of the eighty men evacuating in lifeboats, although there were no reports of them being rescued.

Sortie number four began from Lorient on 29 April 1943; it was also relatively quiet but terminated in Brest on 7 June.

A few days later, (on the 31st), having found U-584, she commenced the re-fuelling operation, but the two boats were spotted by aircraft from the escort carrier USS Card.