SM UB-18 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 21 August 1915. She was commissioned into the German Imperial Navy on 11 December 1915 as SM UB-18. The submarine sank 128 ships in 31 patrols for a total of 130,841 GRT and 725 tons, making her the 17th most successful U-boat in both world wars. UB-18 was rammed by the trawler Ben Lawer and sunk in the English Channel at 49°17′N 5°47′W on 9 December 1917.

The UB II type submarine was a class of U-boat built during World War I by the German Imperial Navy. They were enlarged from the preceding type UB I and were more effective vessels. The boats were a single hull design with a 50-metre maximum diving depth and a 30-45 second diving time. In 1915 and 1916, 30 were built at two different shipyards.