Robert Lyster Thornton (29 November 1908 – 28 September 1985) was a British-Canadian-American physicist who worked on the cyclotrons at Ernest Lawrence's Radiation Laboratory in the 1930s. During World War II he assisted with the development of the calutron as part of the Manhattan Project. He returned to Berkeley in 1945 to lead the construction of the 184-inch (470 cm) cyclotron, and spent the rest of his career there.