Lowell Observatory is an astronomical observatory in Flagstaff, Arizona. Lowell Observatory was established in 1894, placing it among the oldest observatories in the United States, and was designated a National Historic Landmark in 1965.

The Observatory's original 24-inch (0.61 m) Alvan Clark Telescope is still in use today for public education. Lowell Observatory hosts 80,000 visitors per year at their Steele Visitors Center who take guided daytime tours and view various wonders of the night sky through the Clark Telescope and other telescopes. It was founded by astronomer Percival Lowell, and run for a time by his third cousin Guy Lowell of Boston's well-known Lowell family. The current trustee of Lowell Observatory is William Lowell Putnam III, grandnephew of founder Percival Lowell and son of long-time trustee Roger Putnam. The position of trustee is historically handed down through the family.

The observatory operates several telescopes at two locations in Flagstaff. The main facility, located on Mars Hill just west of downtown Flagstaff, houses the original 24-inch (0.61 m) Clark Refracting Telescope, although its role today is as a public education tool and not research. The telescope, built in 1896 for $20,000, was assembled in Boston by Alvan Clark and then shipped by train to Flagstaff. Also located on the Mars Hill campus is the 13-inch (0.33 m) Pluto Discovery Telescope, used by Clyde Tombaugh in 1930 to discover the dwarf planet Pluto.