Rainwater, James (1917-1986) was an American physicist who did important research and theoretical work on the structure of the atomic nucleus. For this work, he shared the 1975 Nobel Prize in physics with Danish physicists Aage Niels Bohr and Ben Roy Mottelson.

Leo James Rainwater was born on Dec. 9, 1917, in Council, Idaho. His parents were Leo Jasper Rainwater, a civil engineer and Edna Eliza Teague Rainwater, manager of a general store. After his father's death in 1918, Rainwater's family moved to Hanford, California. Rainwater excelled in a chemistry competition sponsored by the California Institute of Technology (Caltech) and the school accepted him. He graduated in 1939 as a physics major. He went on to study at Columbia University in New York City.

From 1942–1946, Rainwater worked on the Manhattan Project, the top-secret program that developed the atomic bomb. His main work on the project involved using Columbia's cyclotron to investigate the behavior of atomic nuclei under neutron bombardment. He received a Ph.D. degree in physics from Columbia in 1946. In 1952, he became professor of physics at Columbia.

Rainwater used various techniques of spectroscopy (the observation of the radiation emitted by atoms and molecules) to reveal the structure of atoms and molecules. This work led Rainwater, Bohr, and Mottelson to develop a new theory of the structure of the nucleus. At the time, two theories were used to describe the atomic nucleus. In one, the nuclear particles were arranged in concentric shells. In the other, the nucleus was described as similar to a liquid drop. Rainwater produced a model combining the two ideas. The new theory suggested that the nucleus of an atom is shaped more like a football than a sphere. Rainwater's theory was confirmed experimentally by Bohr and Mottelson, and provided a more complete description of nuclear structure.

Rainwater became a member of the National Academy of Sciences, the Institute of Electrical and Electronics Engineers, the American Association for the Advancement of Science, and the American Physical Society.