is a census-designated place (CDP) in Alameda County, California, United States. As of the 2000 census, it is the fifth most populous unincorporated area in California, and the twenty-third in the United States.
With the arrival of Europeans, Castro Valley was part of the land granted to Mission San Jose in 1797. The area Castro Valley now occupies was part of the extensive colony of New Spain in what was the state of Alta California.
Castro Valley is named after Don Guillermo Castro, who was a soldier in the Mexican army and a rancher. Castro Valley was part of the original
land grant given to Castro, called Rancho San Lorenzo. This land grant included Hayward, San Lorenzo, and Castro Valley, including Crow Canyon, Cull Canyon, and Palomares Canyons. Castro had a gambling habit and had to sell off portions of his land to pay gambling debts. The last of his holding was sold in a sheriff's sale in 1864 to Faxon Atherton for $400,000.
Atherton (after whom the city of Atherton is named) in turn began selling off his portion in smaller parcels. Two gentlemen named Cull (the namesake of Cull Canyon) and Luce bought some
and began running a steam-operated saw mill in Redwood Canyon. The Jensen brothers also bought land from Atherton in 1867.
During the 1940s and 1950s, Castro Valley was known for its chicken ranches. Later it developed into a bedroom community, where workers live and commute to their jobs in the surrounding communities.