Castro Valley is a census-designated place (CDP) located in Alameda County, California, in the United States. It serves as one of the region's significant residential areas and was identified as the fourth most populous unincorporated area in California during the 2020 census.
The population of Castro Valley was recorded at 66,441 in the 2020 census. As an unincorporated community, it maintains its own distinct identity within the broader administrative framework of Alameda County.