San Andreas Lake, Reservoir in Millbrae, California, US.

San Andreas Lake is a reservoir on the San Andreas Fault between Millbrae and San Bruno in San Mateo County. Its water surface covers 220 hectares and stretches 4.8 kilometres at its longest, held back by a 30-metre earth dam. The lake lies at 138 metres above sea level and holds about 23.5 million cubic metres of water.

The Portolá expedition passed the valley on November 4, 1769, before discovering San Francisco Bay; on November 30, 1774, Padre Palóu renamed it Cañada de San Andrés. The natural sag pond was enlarged by a 100-foot (30 m) earth dam in 1868, which withstood the 1906 earthquake although the fault runs beneath it. In 1895 Andrew Cowper Lawson named the San Andreas Fault after the lake.

The valley and creek were named Cañada de San Andrés by Padre Francisco Palóu on 30 November 1774, the feast day of St. Andrew. In 1895 Andrew Cowper Lawson of the University of California, Berkeley, named the fault after the lake. The lake itself is closed as a public water source, so the shore path is the only way to see it.

The reservoir is closed to the public as a drinking-water source, but hikers and cyclists may use a paved trail along the eastern shore just west of Skyline Boulevard, running from San Bruno Avenue in San Bruno to Hillcrest Boulevard in Millbrae. The Sawyer Camp Trail crosses the rift valley on the San Andreas Dam and continues about 6 miles (10 km) to Lower Crystal Springs Reservoir and Crystal Springs Dam.

The site lies directly on the tectonic fault that runs through California, and the dam construction accounts for underground movement. Geologists use the surroundings to study earth plates and fracture lines.