Mount Diablo is a mountain in Contra Costa County, California in the San Francisco Bay Area, located south of Clayton and northeast of Danville. It is an isolated upthrust peak of 3,864 feet (1,178 m), visible from most of the San Francisco Bay Area and much of northern California. Mount Diablo appears from many angles to be a double pyramid and includes many subsidiary peaks, the largest and closest of which is the other half of the double pyramid, North Peak, nearly as high in elevation at 3,557 feet (1,084 m) and about one mile northeast of the main summit.

The peak is the centerpiece of Mount Diablo State Park, a state park of about 20,000 acres (8,000 ha) in area. The park was the first public open space of a complex—according to Save Mount Diablo—now including 38 preserves, including nearby city open spaces, regional parks, watersheds, etc., buffered in some areas with private lands protected with conservation easements. Preserved lands on and around Mount Diablo total more than 90,000 acres (36,000 ha). The day use fee for Mount Diablo State Park is $10 per vehicle.