San Luis Reservoir, Water reservoir in Merced County, California.

San Luis Reservoir is a water storage facility in Merced County, California, that stretches across the eastern slopes of the Diablo Range. The artificial body of water reaches a length of about 9 miles (14 km) and a width of about 5 miles (8 km), surrounded by dry hills and grassland.

Completed in 1967 on land formerly part of Rancho San Luis Gonzaga, the reservoir is a joint facility of the California State Water Project and the federal Central Valley Project. Water is pumped uphill from the O'Neill Forebay, fed by the California Aqueduct, and released back to continue downstream. The San Luis Dam, also called B.F. Sisk Dam, closes the eastern end.

The reservoir supplies farms in the Central Valley with water and plays a key role in California's water distribution system. Visitors often see large pumps and pipelines that belong to the infrastructure and emphasize the technical character of the facility.

The recreation area offers multiple entry points with parking lots, campgrounds, and picnic areas for visitors. Anglers and boaters find ramps and docks along the shoreline, though conditions may vary depending on water levels.

A hydroelectric plant uses the elevation difference between the main basin and a smaller forebay to generate electricity for the nearby town of Los Banos. Water is pumped in both directions depending on demand, allowing the reservoir to function as an energy storage system.