She was commissioned into the German Imperial Navy on 10 February 1918 as SM UB-121.

[Note 1] UB-121 was surrendered to the Allies at Harwich on 20 November 1918 in accordance with the requirements of the Armistice with Germany.

She was transferred to France in 1919, but while in tow to Brest in company with U-118 she ran aground at Birling Gap on 15 April 1919.

Like all Type UB III submarines, UB-121 carried 10 torpedoes and was armed with a 8.8 cm (3.46 in) deck gun.

UB-121 would carry a crew of up to 3 officer and 31 men and had a cruising range of 7,280 nautical miles (13,480 km; 8,380 mi).