SM U-166 was one of the 329 submarines serving in the Imperial German Navy. She was a Type Mittel U submarine, launched in 1918. She would have been engaged in naval warfare and taken part in the First Battle of the Atlantic, but it was not commissioned until the end of the war. She surrendered on 21 March 1919 and became the French submarine Jean Roulier until 24 July 1935. U-166 was broken up.