San Mateo County is located in the San Francisco Bay Area of the U.S. state of California. It covers most of the San Francisco Peninsula just south of San Francisco. San Francisco International Airport is located at the northern end of the county, and Silicon Valley begins at the southern end. As of 2005 the population was 712,462. The county seat is Redwood City.
San Mateo County was formed in 1856 and bears the Spanish name for Saint Matthew. The county has a total area of 741 mi², 61% of it is land and 39% of it is water. As of 2000 the population density was 1,575/mi².