Sherwood Dam, Gravity dam in Ventura County, United States.

Sherwood Dam is a 270-foot-long concrete arch dam in the Santa Monica Mountains near Thousand Oaks, holding back the 165-acre Lake Sherwood. Also called Alturas Dam and Potrero Dam, it was completed in 1904 and remains one of California's oldest standing dams.

W. H. Matthiessen, a rancher who owned the land, led construction of the Alturas Dam, completed in 1904; the 165-acre Potrero Lake formed the following winter. Matthiessen's son Christy let Douglas Fairbanks film Robin Hood there in 1921, and the ranch became known as Sherwood Forest. Elsie Canterbury sold the land west of the lake to William Randolph Hearst, who died in 1951.

The lake earned its name during 1920s film productions of Robin Hood with Douglas Fairbanks, when the location served as a stand-in for Sherwood Forest. This movie connection shaped how people think of the place and remains woven into local identity today.

The dam and Lake Sherwood sit in a private residential community; there is no public swimming, fishing or boating access. The structure is 270 feet long and 287 metres above sea level, about 3.9 km southwest of Thousand Oaks. After its 1984 reinforcement it remains susceptible to 100-year floods and high-magnitude earthquakes.

The dam's reservoir prompted William Mulholland to begin designing the Los Angeles Aqueduct in 1905, and the Malibou Lake Mountain Club built a similar dam nearby in 1922. In 1984 the lake was drained so the dam could be inspected and reinforced.