Sir John Douglas Cockcroft (1897–1967) was an English physicist.
Cockroft was a fellow of St. John's College, Cambridge (1928–46) and professor of natural philosophy at the University of Cambridge (1939–46). The 1951 Nobel Prize in Physics was awarded jointly to Cockcroft and E.T.S. Walton for their pioneer work in transmuting atomic nuclei by bombarding elements with artificially accelerated atomic particles. He was knighted in 1948. Today, a building named after him exists in the University of Cambridge, comprising a lecture theatre and several hardware laboratories.