English mathematician and physicist noted for his research in geophysics.

Chapman was educated at Victorian University of Manchester and at Trinity College, Cambridge. One of his earliest scientific contributions was to modify Maxwell's kinetic theory of gases, thereby predicting the phenomenon of thermal diffusion and later confirming it experimentally (1912–17). His interest in geophysics was aroused while he served as chief assistant (1910–14, 1916–19) to Sir Frank W. Dyson, the Astronomer Royal at Greenwich, where he helped design a new magnetic observatory. This involvement led him to study magnetic storms and variations in the Earth's magnetic field, discovering that the geomagnetic field is at least partly generated in the atmosphere.

Chapman's later work included studies of auroras, magnetic disturbances of the ionosphere, and thermal diffusion in ionized gases. Chapman was elected to many learned societies, including the Royal Society in 1919, and for his contributions to understanding geomagnetism he received the Royal Society's Copley Medal in 1964.