San Gabriel Mountains National Monument, Nature reserve in San Bernardino County, United States.

San Gabriel Mountains National Monument covers 346,177 acres of the San Gabriel Mountains in Los Angeles and San Bernardino counties, managed by the U.S. Forest Service within Angeles and San Bernardino National Forests. It spans from Little Tujunga Canyon Road to Telegraph Peak and includes the Sheep Mountain, San Gabriel and Pleasant View Ridge wilderness areas. Mount San Antonio, at 3,069 meters the range's highest peak, rises inside the monument.

In 1891 President Benjamin Harrison designated the San Gabriel Timberland Reserve, the first federal forest protection in the United States, after conservationists Abbot Kinney and John Muir influenced Congress. In 2003 Congresswoman Hilda Solis commissioned a feasibility report on monument status. President Barack Obama proclaimed the 346,177-acre monument on October 10, 2014, and in May 2024 President Joe Biden expanded it by 105,919 acres.

Archaeological finds in the monument date back up to 8,000 years, and more than 600 archaeological sites are known, three of them listed in the National Register of Historic Places. Ruins of old settlers' cabins and of the Mount Lowe Railway also stand in the area, and the ghost town of Eldoradoville lies on the San Gabriel River.

The reserve has numerous trails suitable for hiking and mountain biking at various difficulty levels, with well-known peaks like Mount San Antonio and Mount Baden-Powell accessible to visitors. Come prepared with plenty of water and wear layers, as weather can change quickly at higher elevations.

The monument supplies about 30 percent of Los Angeles's drinking water and holds more than 300 endemic plant species. It also contains 87 miles of the Pacific Crest Trail and the Mount Wilson Observatory, and archaeologists have recorded over 600 sites, some up to 8,000 years old.