Belmont, California in the context of Ralston Hall

San Mateo is the most populous city in San Mateo County, California, United States, on the San Francisco Peninsula. It is part of the San Francisco Bay Area metropolitan region, and is located about 20 miles (32 km) south of San Francisco. San Mateo borders Burlingame to the north, Hillsborough to the west, San Francisco Bay and Foster City to the east and Belmont to the south. The population was 105,661 at the 2020 census. Some of the biggest economic drivers for the city include technology, health care and education.

The San Francisco Peninsula is a peninsula in the San Francisco Bay Area that separates San Francisco Bay from the Pacific Ocean. On its northern tip is the City and County of San Francisco. Its southern base is Palo Alto, Mountain View, Los Altos and Sunnyvale, in Santa Clara County. Most of the Peninsula is occupied by San Mateo County, which is between San Francisco and Santa Clara counties, and the Peninsula includes the cities and towns of Atherton, Belmont, Brisbane, Burlingame, Colma, Daly City, East Palo Alto, El Granada, Foster City, Half Moon Bay, Hillsborough, La Honda, Loma Mar, Los Altos, Los Altos Hills, Menlo Park, Millbrae, Mountain View, Pacifica, Palo Alto, Pescadero, Portola Valley, Redwood City, Redwood Shores, San Bruno, San Carlos, San Mateo, South San Francisco, Sunnyvale, West Menlo Park and Woodside.

Whereas the term peninsula in a geographical sense technically refers to the entire San Francisco Peninsula, in local jargon, "The Peninsula" does not include the city of San Francisco.