John Cromwell Mather (b. August 7, 1946, Roanoke, Virginia) is an American astrophysicist, cosmologist and Nobel Prize in Physics laureate for his work on the Cosmic Background Explorer Satellite (COBE) with George Smoot. COBE was the first experiment to measure "... the black body form and anisotropy of the cosmic microwave background radiation."

This work helped cement the big-bang theory of the universe. According to the Nobel Prize committee, "the COBE-project can also be regarded as the starting point for cosmology as a precision science."

Mather is a senior astrophysicist at the U.S. space agency's (NASA) Goddard Space Flight Center in Maryland and adjunct professor of physics at the University of Maryland, College Park. In 2007, Mather was listed among Time magazine's 100 Most Influential People in The World.

Mather is also the project scientist for the James Webb Space Telescope, a space telescope to be launched to L2 no earlier than 2018.

In October 2010 Mather will be participating in the USA Science and Engineering Festival's Lunch with a Laureate program where middle and high school students will get to engage in an informal conversation with a Nobel Prize winning scientist over a brown bag lunch. Mather is also a member of the USA Science and Engineering Festival's Advisory Board.