San Mateo County (/ˌsæn məˈteɪ.oʊ/ SAN mə-TAY-oh; Spanish for "Saint Matthew") is a county in the U.S. state of California. As of the 2020 census, the population was 764,442.[1] The county seat is Redwood City.[2] The county is in the San Francisco Bay Area region. It is on the San Francisco Peninsula.