Gregory Breit (Ukrainian: Григорій Альфредович Брейт-Шнайдер, Russian: Григорий Альфредович Брейт-Шнайдер, romanized: Grigory Alfredovich Breit-Shneider; July 14, 1899 – September 13, 1981) was an American physicist born in Mykolaiv, Russian Empire (now Mykolaiv, Ukraine).

In 1925, while at the Carnegie Institution of Washington, Breit joined with Merle Tuve in using a pulsed radio transmitter to determine the height of the ionosphere, a technique important later in radar development.

[3] Together with Eugene Wigner, Breit gave a description of particle resonant states with the relativistic Breit–Wigner distribution in 1929, and with Edward Condon, he first described proton-proton dispersion.

In April 1940, he proposed to the National Research Council that American scientists observe a policy of self-censorship due to the possibility of their work being used for military purposes by enemy powers in World War II.

In 2014, experimentalists proposed a way to validate an idea by Breit and John A. Wheeler that matter formation could be achieved by interacting light particles ("Breit–Wheeler process").