Lopez Lake, Recreation lake in San Luis Obispo County, California, US.

Lopez Lake is a reservoir near Arroyo Grande in San Luis Obispo County, California, formed by a dam built across Arroyo Grande Creek. The lake sits among rolling hills and dry canyons, with a shoreline that winds through several coves used for boating, swimming, and fishing.

The Lopez Dam was completed in 1969 to supply drinking water to several towns in San Luis Obispo County and to support farming in the Arroyo Grande Valley. Over the following decades, the area around the reservoir was gradually developed into a recreation zone open to the public.

Lopez Lake hosts the Lopez Lake Trout Derby each May, and the park also stages the California Polytechnic University Triathlon and the Scott Tinley Dirty Adventures Triathlon. The reservoir is unusual among municipal water supplies in allowing human contact with the water, so sailing, windsurfing, water skiing, swimming and fishing all take place on it.

Lopez Lake lies 9 miles (14 km) upstream from the Pacific Ocean on Arroyo Grande Creek, so the drive from Arroyo Grande follows the creek valley inland. The 4,200-acre recreation area holds trails and camping areas, and the lake is one of the few municipal water supplies where contact with the water is allowed. Water is piped 3 miles (4.8 km) to a terminal reservoir before treatment.

In March 2023 the reservoir reached its capacity of 49,200 acre-feet and spilled over the spillway for the first time since 1998, about 25 years earlier. The earth-fill dam itself was retrofitted between 2001 and 2003 to resist earthquakes.