San Mateo County, officially the County of San Mateo, is a county located in the U.S. state of California. 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 (metropolitan statistical area), Silicon Valley, and is part of the San Francisco Bay Area, the nine counties bordering San Francisco Bay. As of 2020 CE, it has a median household income of $128,091, the fourth-highest household income of any county in the nation behind Loudoun County, Virginia, Falls Church, Virginia, and Santa Clara County, California.
The county covers most of the San Francisco Peninsula. The county is predominantly suburban and is home to many corporate campuses.