Lake Berryessa is the largest lake in Napa County, California. This reservoir is formed by the Monticello Dam, which provides water and hydroelectricity to the North Bay region of the San Francisco Bay Area.

Prior to its inundation, the Beryessa Valley was an agricultural region, whose soils were considered among the finest in the country. The main town in the valley, Monticello, was abandoned in order to construct the reservoir. This abandonment was chronicled by the photographers Dorothea Lange and Pirkle Jones in their book Death of a Valley. Construction of Monticello Dam was begun in 1953,and the reservoir filled by 1963, creating what at the time was the second-largest reservoir in California after Shasta Lake.

The lake is heavily used for recreational purposes and encompases over 20,000 acres (80 km²) when full. The reservoir is approximately 15.5 miles (25 km) long, but only 3 miles (5 km) wide. It has approximately 165 miles (265 km) of shoreline. It has a seaplane landing area that is open to the public. One of the larger islands supported a small plane landing area, but was closed down in the early 1970s after the FAA issued a safety report.. The lake is fed by the head waters to the 576 square mile (1,490 km²) Putah Creek watershed. It has a storage capacity of 1,602,000 acre feet (1.98 km³), making it one of the larger reservoirs in California. Though the land behind the Monticello Dam is in Napa County, the water contained above is essentially owned by Solano County. The water is used for hydroelectric and agricultural purposes outside of Napa County, CA.

Lake Berryessa is a swimming and water skiing site for enthusiasts. The narrow portion of the reservoir, nearest to the Monticello Dam, is referred to as the "Narrows," and is sometimes busy with boaters on holidays and weekends.