German submarine U-81 was a Type VIIC U-boat of the navy (Kriegsmarine) of Nazi Germany during World War II, famous for sinking the aircraft carrier HMS Ark Royal.

The submarine was powered by two MAN M 6 V 40/46 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.

She then became a front (operational) boat of the 1st U-boat Flotilla, and set out on a number of training patrols.

Her first attempt to break into the 'Italian Lake' almost ended in disaster, when on 30 October she was attacked and severely damaged by a British Catalina of No.

The Catalina was joined by a Lockheed Hudson, which dropped depth charges onto U-81 causing severe damage and forcing her to return to Brest.

On 13 November off Gibraltar, she encountered the inbound ships of Force H. She fired a single torpedo into the aircraft carrier HMS Ark Royal, and then avoided depth charge attacks from the escorts.

The U-boat put into port at Salamis in Greece on 25 April, having spent 22 days at sea and sunk 7,582 GRT of shipping.

Her next patrol sank three more Egyptian sailing vessels, the Bourghieh, the Mawahab Allah and the Rousdi.

Her next patrol saw only the Empire Moon hit on 22 July, but she was declared a total loss and spent the rest of the war under repair.

The U-boat's next three patrols were uneventful but on 18 November she sank the cargo ship Empire Dunstan.