Diamond Valley Lake is a man-made off-stream reservoir located near Hemet, California, United States. It is one of the largest reservoirs in Southern California and one of the newest. It has a capacity of 800,000 acre-feet (990,000,000 m3). The lake nearly doubled the area's surface water storage capacity and provides additional water supplies for drought, peak summer, and emergency needs.

Hemet is a city in the San Jacinto Valley in Riverside County, California. It covers a total area of 29.3 square miles (76 km2), about half of the valley, which it shares with the neighboring city of San Jacinto. The population was 89,833 at the 2020 census.