Mount San Antonio, commonly referred to as Mount Baldy or Old Baldy, is a 10,064 ft (3,068 m) summit in the San Gabriel Mountains on the border of Los Angeles and San Bernardino counties of California. Lying within the San Gabriel Mountains National Monument and Angeles National Forest, it is the highest point within the mountain range, the County of Los Angeles, and the Los Angeles metropolitan area.

Mount San Antonio's sometimes snow-capped peaks are visible on clear days and dominate the view of the Los Angeles Basin skyline. The peak and a subsidiary one to the west form a double summit of a steep-sided east–west ridge. The summit is accessible via a number of connecting ridges along hiking trails from the north, east, south, and southwest.

The Tongva call the mountain Yoát or Joat, which means snow, and the Mohave call it Avii Kwatiinyam. The name Mount San Antonio was probably bestowed by Antonio Maria Lugo, owner of a rancho near present-day Compton circa 1840, in honor of his patron saint, Anthony of Padua.

The mountain is almost always referred to as "Mount Baldy" by locals, to the point where many may not recognize the name "Mount San Antonio." When American settlers arrived and surveyed the land, "Baldy" – a reference to the bare fell-field of Baldy Bowl that dominates the south face visible from Los Angeles – became the predominant name, and it has stuck. Nonetheless, "Mount San Antonio" is still used by a number of institutions (e.g. Mount San Antonio College).

Mt. San Antonio lies in the Angeles National Forest. Its summit is 10,064 feet (3,068 m), and marks the boundary between San Bernardino County and Los Angeles County. A sub-peak, West Baldy, is 10,002 feet (3,049 m).

The mountain's southern watershed drains into San Antonio Creek, the north side into Lytle Creek and the Fish Fork of the San Gabriel River. San Antonio and Lytle Creeks are part of the Santa Ana River watershed. San Antonio Creek descends through a deep canyon which has several waterfalls, the last about 75 feet (23 m) high.

East of the summit is Mount Harwood, which is in turn connected by a narrow ridge, "The Devil's Backbone," to a pass known as the Baldy Notch. At the Notch there is a ski resort, the closest one to Los Angeles. South of the resort, and connected to its ski lift by an asphalt road, lies Mt Baldy Village.

There are no roads or maintained trails connecting the mountain to the less populated region to its north, but a use trail leads over Dawson Peak and Pine Mountain to Wright Mountain and the Pacific Crest Trail, overlooking the town of Wrightwood.