Castle Mountains National Monument, Desert nature reserve in San Bernardino County, California.

Castle Mountains National Monument protects 20,920 acres of the Castle Mountains, a southwest-to-northeast range reaching 5,543 feet at Hart Peak and 5,580 feet at Linder Peak. The monument lies between Interstates 15 and 40, northwest of the Colorado River, and is bordered on three sides by Mojave National Preserve. It surrounds the 8,340-acre Castle Mountain Mine Area, an open-pit gold mine.

President Barack Obama designated Castle Mountains National Monument on February 12, 2016, together with Mojave Trails and Sand to Snow. Of the three, only Castle Mountains is managed by the National Park Service; the others went to the Bureau of Land Management or the U.S. Forest Service. The monument surrounds the 8,340-acre Castle Mountain Mine Area, an open-pit gold mine suspended since 2001, whose federal land is to pass to the Park Service after mining and reclamation end, or after ten years without mining.

Archaeological sites scattered across the land show that people have lived here for a very long time. The remains of an old mining settlement reveal how people once worked and lived in this harsh desert environment.

Visitors need tough vehicles with four-wheel drive and high ground clearance to explore the area. Unpaved roads wind through the monument and conditions can change with weather.

The grasslands at the base of the mountains are home to Joshua tree forests and plants found nowhere else. These rare habitats give visitors a chance to see ecosystems that exist only in this part of the desert.