San Mateo County, officially the County of San Mateo, is a county in the U.S. state of California. As of the 2020 census, the population was 764,442. Redwood City is the county seat, the third-most populated city in the county after Daly City and San Mateo. San Mateo County is included in the San Francisco–Oakland–Berkeley, CA MSA, Silicon Valley, and is part of the San Francisco Bay Area, the nine counties bordering San Francisco Bay.