William Kenneth Hartmann (born June 6, 1939) is an American planetary scientist, artist, author, and writer.

He has long been one of America's leading space artists (strongly influenced by Chesley Bonestell), and has written and illustrated numerous books on the history of Earth and the Solar System, often in collaboration with artist Ron Miller.

His written work also includes textbooks, short fiction, and novels, the most recent being published in 2003.

Hartmann was a member of the 1966–1968 University of Colorado UFO Project (informally known as the Condon Committee), a controversial[citation needed] public study of UFOs sponsored by the U.S. Air Force.

He primarily investigated photographic evidence, and rejected most as unreliable or inconclusive; in his studies published in the Committee's final report, he concluded two cases - Great Falls (motion pictures of two bright light sources difficult to reconcile with known aircraft) and McMinnville (two photographs of a saucer-shaped craft)[3] - were unexplained and particularly noteworthy as probative evidence of the reality of UFOs.