San Mateo County is a county 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, and north of Santa Clara County.

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. It is among the 20 most affluent counties in the United States, in terms of personal, per capita and household income. It is strongly Democratic and ethnically diverse. The county's built-up areas are mostly suburban with some areas being very urban, and are home to several corporate campuses. San Mateo County was formed from parts of San Francisco County and Santa Cruz County in 1856.