SM UB-84 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 31 October 1917 as SM UB-84. UB-84 was lost in a collision on 7 December 1917 in the Baltic Sea at . She was later raised and used as a training boat. On 26 November 1918 she was surrendered to the French in accordance with the requirements of the Armistice with Germany and broken up in Brest in 1921.