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 earliest known settlers of the Bay Area were the Ramaytush people, since at least 10000 BC.

In 1789, the Spanish missionaries had named a Native American village along Laurel Creek as Los Laureles or the Laurels (Mission Dolores, 1789). At the time of Mexican Independence, 30 native Californians were at San Mateo, most likely from the Salson tribelet.

In 1827, Captain Frederick William Beechey was traveling with the hills on their right, known in that part as the Sierra del Sur, began to approach the road, which passing over a small eminence, opened out upon "a wide country of meadow land, with clusters of fine oak free from underwood... It strongly resembled a nobleman's park: herds of cattle and horses were grazing upon the rich pasture, and numerous fallow‑deer, startled at the approach of strangers, bounded off to seek protection among the hills... This spot is named San Matheo, and belongs to the mission of San Francisco."

The city of San Mateo was documented by Spanish colonists as part of the Rancho de las Pulgas and the Rancho San Mateo; the earliest history is held in the archives of Mission Dolores. Rancho San Mateo was granted in May 1846 from Pío Pico to his secretary, Cayetano Arenas; this tract included approximately half of present-day San Mateo, all of Burlingame and Hillsborough, and the Spring Valley lakes (now Crystal Springs Reservoir and San Andreas Lake). William Davis Merry Howard purchased Rancho San Mateo from Arenas in 1846 for US$25,000 (equivalent to $875,000 in 2024) and spent an equal sum erecting a fence around the property.

In the 1850s, following the American Conquest of California, many San Franciscans began building summer homes in the mid-Peninsula, because of the milder climate. The area that is now the city of San Mateo was owned by a few large landowners, including Howard, whose Rancho San Mateo occupied 6,438 acres (2,605 ha) in 1853 north of San Mateo Creek, pushing most of this early settlement into adjacent Hillsborough, Burlingame, and Belmont; other significant landowners in the area included John Parrott, who purchased 500 acres (200 ha) in 1860 south of the creek and southwest of El Camino Real and Alvinza Hayward, who owned the land south of the creek and east of El Camino. Much of the remaining land south of these areas was used for agriculture until the early 1900s; the owners included John Whipple, who had a large horse farm south of Parrott's land, Lemuel Murray, J.S. Colegrove, and David McClellan.

In 1858, Sun Water Station, a stage station of the Butterfield Overland Mail route, was established in San Mateo. It was 9 miles (14 km) from both Clarks Station (to the north) in what is now San Bruno and the next station south at Redwood City.

Several historically important mansions and buildings were constructed in San Mateo around this time. A.P. Giannini, founder of the Bank of Italy (which later became the Bank of America), lived here most of his life. His mansion, Seven Oaks, is listed in the National Register of Historic Places (No. 99001181). It is located at 20 El Cerrito Avenue.