Horsethief Canyon, Canyon in San Bernardino County, California.

Horsethief Canyon is a canyon in San Bernardino County that forms the upper section of Summit Valley. It descends from about 3,800 feet (1,200 m) near Cajon Pass to 3,123 feet (950 m) at its mouth.

The canyon forms the upper part of Summit Valley, its head at 3,800 feet just east of Cajon Pass and its mouth at 3,123 feet. California State Routes 138 and 173 were later built through this natural corridor. A separate Horsethief Canyon lies near Pine Valley in San Diego County's Cleveland National Forest.

The name is shared with a separate Horsethief Canyon in San Diego County, near Pine Valley and the Mexico–United States border, inside Cleveland National Forest's Pine Creek Wilderness. The San Bernardino County canyon lies at the head of Summit Valley, just east of Cajon Pass, and California State Routes 138 and 173 run through it.

The canyon is accessible via California State Routes 138 and 173, which both pass through it. Visitors can approach from several directions throughout the county.

A separate Horsethief Canyon lies near Pine Valley in San Diego County, within Cleveland National Forest's Pine Creek Wilderness by the Mexico–United States border. This San Bernardino County canyon is a different place, its head just east of Cajon Pass at 3,800 feet.