Castro Valley is a census-designated place in Alameda County, California, United States. At the 2010 census, it was the fifth most populous unincorporated area in California and the twenty-third most populous in the United States. The population was 66,441 at the 2020 census. Castro Valley is named after Don Guillermo Castro, a noted 19th-century Californio ranchero who owned the land where the community is located.