John R. Dunning (1907-1975) was an American physicist.
In 1939, Dunning led the American research team verifying fission of the uranium atom. During the Manhattan Project, Dunning led the day-to-day operations and research at the Substitute Alloy Materials Lab (SAM) at Columbia. He conducted work on gaseous diffusion to separate uranium isotopes. This method was used by the Manhattan Project to separate U-235 from U-238 in the manufacture of the atomic bomb.
After the war, Dunning became a full professor of physics at Columbia in 1946 and directed its engineering faculty.