John Cromwell Mather (born August 7, 1946) is an American astrophysicist, cosmologist and Nobel Prize in Physics laureate for his work on the Cosmic Background Explorer Satellite (COBE) with George Smoot.

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 one of the 20 American recipients of the Nobel Prize in Physics to sign a letter addressed to President George W. Bush in May 2008, urging him to "reverse the damage done to basic science research in the Fiscal Year 2008 Omnibus Appropriations Bill" by requesting additional emergency funding for the Department of Energy's Office of Science, the National Science Foundation, and the National Institute of Standards and Technology.

[2] Mather served as the senior project scientist for the James Webb Space Telescope (JWST) from 1995 until 2023, when he was succeeded by Jane Rigby.

[3] In 2014, Mather delivered an address on the James Webb Space Telescope at the second Starmus Festival in the Canary Islands.