SM UB-20 was a German Type UB II submarine or U-boat in the German Imperial Navy () during World War I. The U-boat was ordered on 30 April 1915 and launched on 26 September 1915. She was commissioned into the German Imperial Navy on 10 February 1916 as SM UB-20. The submarine sank 13 ships in 15 patrols for a total of . UB-20 was mined and sunk on 28 July 1917 at . Thirteen crew members died in the event.