Peter R. McCullough (/məˈkʌlə/; born August 20, 1964) is an American astronomer, founder of the XO Project and discoverer of extrasolar transiting planets, such as XO-1b.

[3] Astronomers John Gaustad, McCullough, and David Van Buren with engineer Wayne Rosing mapped the entire southern sky in the hydrogen alpha transition with sufficient sensitivity for decontamination of the Milky Way from the cosmic microwave background.

McCullough received a Bachelor of Science in physics in 1986 from the University of North Carolina at Chapel Hill.

[6] McCullough was a Hubble postdoctoral fellow, and then an assistant professor in the Astronomy Department of the University of Illinois Urbana-Champaign.

[11][12] NASA granted him a graduate student researcher award and a Hubble postdoctoral fellowship in 1993.