John Ray Dunning (September 24, 1907 - August 25, 1975) was a US physicist who played a key role in the development of the atomic bomb.

Born Shelby, Nebraska and graduating from Nebraska Wesleyan University in 1929, he was awarded his Ph.D. from Columbia University in 1934. He spent the next two years in Europe surveying the emerging field of nuclear physics and returned to Columbia in 1936 to lead the construction of the US's first cyclotron.

In 1938, German physicists Otto Hahn and Fritz Strassmann claimed to have achieved nuclear fission and it was Dunning who led a team to replicate and verify their reports. In 1940, Dunning's team, which enjoyed Alfred O. C. Nier's expertise in mass spectrometry, showed further that it was the comparatively rare uranium-235 that was fissile, rather than the abundant uranium-238. Dunning went on to lead the Columbia team that developed the gaseous diffusion method for separating the two isotopes and which was critical to the Manhattan project to develop the atomic bomb.

Dunning became professor of physics at Columbia in 1946 and was head of the engineering faculty from 1950 to 1969. He died in Key Biscayne, Florida.