Crystal Springs Reservoir, Water reservoir in San Mateo County, California, US.

Crystal Springs Reservoir is an artificial water body located in the northern Santa Cruz Mountains along the San Andreas Fault in San Mateo County, California. The reservoir consists of two large connected lakes that wind through a narrow forested valley between rolling hills covered with woodland and coastal scrub vegetation.

The southern Upper Crystal Springs Reservoir began in 1877, when the Spring Valley Water Company built an earthen dam on Laguna Creek, submerging the natural Laguna Grande. The Portolà Expedition had camped at that lake on November 5, 1769. In 1888 Herman Schussler's 46 m concrete Crystal Springs Dam formed the northern Lower reservoir, drowning the resort town of Crystal Springs; the old earthen dam became the Highway 92 causeway.

The name Laguna Grande survives on a California Historical Marker at the upper lake, and the 1769 Portolà expedition camped here on November 5; Fray Juan Crespí's journal records ducks and geese on the water and Lamchin people bringing black pies and a cherry-like fruit. The Crystal Springs Regional Trail follows the shore.

A trail runs along the eastern shore and provides access to several viewpoints overlooking the water. The route is suitable for walking and cycling, while swimming and direct access to the water are not permitted.

The water comes partly from local rainfall and partly from Hetch Hetchy Reservoir in Yosemite National Park, connected by long-distance pipelines. The San Andreas Fault runs directly beneath the water through the middle of the reservoir.