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.

[1][4] He was one of the early pioneers of the cyclotron, a group that included Bernard Kinsey, Franz Kurie, Edwin McMillan, Arthur Snell and Stanley van Voorhis.

[1][6] He later bemoaned the fact that little valuable physics was done owing to the Radiation Laboratory's preoccupation with the cyclotrons and the detectors,[7] but he was involved in an exploration of the Oppenheimer–Phillips process.

[1][10] This was a device for uranium enrichment using electromagnetic separation, as part of the Manhattan Project, the effort to develop atomic bombs during World War II.

[10] After the war ended he returned to Washington University in St. Louis, where Arthur H. Compton was building up the physics department.

Under a special arrangement between Lawrence and Robert Gordon Sproul, the president of the University of California, Thornton was made a professor of physics at the Radiation Laboratory.

Thornton became a regular professor at the University of California in 1948, and started teaching courses in mechanics, and electricity and magnetism.