Lac de Saint-Cassien, Artificial reservoir in Provence-Alpes-Côte d'Azur, France.

Lac de Saint-Cassien is an artificial reservoir in the Provence-Alpes-Côte d'Azur region of southern France, located within the communes of Tanneron, Montauroux, and Callian. The body of water covers roughly 420 hectares and holds a volume of around 60 million cubic meters.

Built between 1962 and 1965 and put into service in 1966, the Saint-Cassien dam is an earth-fill embankment rising to 158.50 m NGF. It was planned after the collapse of the Malpasset dam on 2 December 1959 to secure water supplies for the south-east of Var and the far south-west of Alpes-Maritimes, fed mainly by a diversion canal from the Siagne. The reservoir drowned seven kilometres of the Roman aqueduct from Mons to Fréjus, in which a concrete pipe had been laid in 1894.

The name honors John Cassian, an early monk and theologian who founded the Abbey of Saint Victor in Marseille during the fifth century. Today the lake draws families and water sports enthusiasts who swim, picnic along its shores, or paddle kayaks across the calm surface.

The reservoir offers several designated swimming areas with access to pedal boat rentals, canoes, and kayaks. Anglers will find dedicated spots along the shores where they can set up their gear.

The Fondurane bird reserve, created in 1988, covers 43 hectares in the western part of the lake and has been managed by the C.E.E.P. (Conservatoire Études des Écosystèmes de Provence) ever since. It is a separate protected area from the reservoir itself, which was filled between 1962 and 1965.