Castro Valley is a census-designated place in Alameda County, California, United States. As of the 2010 census, it is the fifth most populous unincorporated area in California and the twenty-third most pop…Castro Valley is a census-designated place in Alameda County, California, United States. As of the 2010 census, it is 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.
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