Mount Lemmon Observatory (MLO), also known as the Mount Lemmon Infrared Observatory, is an astronomical observatory located on Mount Lemmon in the Santa Catalina Mountains approximately 28 kilometers (17 miles) northeast of Tucson, Arizona (US).

The site in the Coronado National Forest is used with special permission from the U.S. Forest Service by the University of Arizona's Steward Observatory, and contains a number of independently managed telescopes.

[1] The MLO site was first developed in 1954 as Mount Lemmon Air Force Station, a radar installation of the Air Defense Command.

[1] Until 2003, a radar tower operated from Fort Huachuca was used to track launches from the White Sands Missile Range in New Mexico and Vandenberg Air Force Base in California.

[3] Below are the 8 telescopes currently operating at the observatory.