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Sir James Chadwick (20 October 1891 – 24 July 1974) was an English physicist who was awarded the 1935 Nobel Prize in physics for his discovery of the neutron in 1932. He was the head of the British scientists who worked on the Manhattan Project during World War II. He was knighted in 1945 for achievements in physics.

A graduate of the University of Manchester, where he studied under Ernest Rutherford, Chadwick was awarded an 1851 Exhibition Scholarship, and elected to study beta radiation under Hans Geiger. Using Geiger's recently developed Geiger counter, he was able to demonstrate that beta radiation produced a continuous electromagnetic spectrum, and not discrete lines as had been thought. Still in Germany when World War I broke out in Europe, he spent the war in the Ruhleben internment camp.

After the war, Chadwick followed Rutherford to the Cavendish Laboratory at the University of Cambridge, where Chadwick earned his Doctor of Philosophy degree at Gonville and Caius College, Cambridge. He was Rutherford's Assistant Director of Research at the Cavendish Laboratory for over a decade at a time when it was one of the world's foremost centres for the study of physics, attracting students like John Cockcroft, Norman Feather, and Mark Oliphant. In 1932, Chadwick pursued a line of research that led to his discovery of the neutron. He went on to measure its mass.

Chadwick left the Cavendish Laboratory in 1935 to become a professor of physics at the University of Liverpool, where he overhauled an antiquated laboratory and, by installing a cyclotron, made it an important centre for the study of nuclear physics. During the Second World War, he carried out research as part of the Tube Alloys project to build an atomic bomb. When the Quebec Agreement merged his project with the American Manhattan Project, he became head of the British Mission, and worked at the Los Alamos Laboratory. In July 1945, he viewed the Trinity nuclear test. Uncomfortable with the trend toward Big Science, Chadwick became the Master of Gonville and Caius College, Cambridge, in 1948. He retired in 1959.