Mount San Antonio, Mountain summit on the Los Angeles–San Bernardino county line, California, United States

Mount San Antonio, also called Mount Baldy, is a 10,064-foot (3,068 m) double summit on the Los Angeles–San Bernardino county line in the San Gabriel Mountains. Its steep-sided east–west ridge rises above Angeles National Forest, with a bare south face and a snow-covered crest in the photograph.

Antonio Maria Lugo, a rancho owner, named the mountain around 1840 after Anthony of Padua. The area later saw the development of hiking and recreational facilities by local mountaineering groups.

The peak carries the nickname "Mount Baldy" because of its treeless summit visible from many parts of Southern California. Hikers and climbers use the area year-round, with conditions changing dramatically by season.

Several hiking routes lead to the summit, with access varying by weather and snow cover. Visitors should start early in the morning to avoid afternoon thunderstorms common during summer months.

Between 1922 and 1927 the physicist Albert A. Michelson ran a series of experiments here, bouncing a light beam to a reflector on Lookout Mountain; the known distance between the two points gave a speed of light of 299,796±4 km/s. The actor Julian Sands disappeared after setting out on a hike here on 13 January 2023, and a body found days earlier was identified as his in late June 2023.