James Watson Cronin (born September 29, 1931) is an American nuclear physicist.
Cronin was born in Chicago, Illinois and attended Southern Methodist University in Dallas, Texas. Cronin and co-researcher Val Logsdon Fitch were awarded the 1980 Nobel Prize in Physics for a 1964 experiment that proved that certain subatomic reactions do not adhere to fundamental symmetry principles. Specifically, they proved, by examining the decay of kaons, that a reaction run in reverse does not merely retrace the path of the original reaction, which showed that the interactions of subatomic particles are not indifferent to time. Thus the phenomenon of CP violation was discovered.
Cronin is Professor Emeritus at the University of Chicago and a spokesperson for the Auger project. Cronin is a member of the Board of Sponsors of The Bulletin of the Atomic Scientists