Diablo is a census-designated place (CDP) in Contra Costa County, California, United States. The population was 1,158 at the 2010 census. It is located 2.5 miles (4.0 km) east-northeast of Danville. "Diablo" is Spanish for devil.
According to the United States Census Bureau, the CDP has a total area of 1.4 square miles (3.6 km2), all of it land.
A post office was established at Diablo in 1916.