SM UB-121 was a German Type UB III submarine or U-boat in the German Imperial Navy () during World War I. She was commissioned into the German Imperial Navy on 10 February 1918 as SM UB-121. UB-121 was surrendered to France on 20 November 1918 in accordance with the requirements of the Armistice with Germany. She was broken up in Toulon in July 1921 after being used for underwater demolition training.