Like all Type UB III submarines, UB-81 carried 10 torpedoes and was armed with a 8.8 cm (3.46 in) deck gun.

On the night of 30 November/1 December 1917 she torpedoed and sank the 3,218 ton British steamer Molesey 12 miles west-south-west of the Brighton Light Vessel.

[6] UB-81 struck a mine on the night of 2 December 1917 in the English Channel to the southeast of the Isle of Wight off Dunnose Head.

The crew of 34, commanded by Oberleutnant zur See Reinhold Saltzwedel, managed to raise the forward torpedo tubes above the surface and seven crewmen escaped before a collision occurred with a British patrol boat and she sank; another source claims that 35 men were aboard and that six survived.

[8][9] The wreck is designated as a controlled site under the Protection of Military Remains Act 1986[10] and therefore all diving on her is strictly prohibited.