SM UB-21 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 20 February 1916 as SM UB-21. The submarine sank 35 ships in 26 patrols for a total of . Surrendered to Britain in accordance with the requirements of the Armistice with Germany, UB-21 was lost in 1920 off the British East coast while en route to be broken up.