Lake Annecy is a perialpine lake in Haute-Savoie in France. It is named after the city of Annecy, which marks the start of the Thiou, Lake Annecy's outflow river.
It is the third-largest lake in France, after the Lac du Bourget and Lac de Grand-Lieu, if the French part of Lake Geneva, which is shared between Switzerland and France, is excluded. It is known as "Europe's cleanest lake" because of strict environmental regulations introduced in the 1960s. It is a popular tourist destination known for its swimming and water sports.
The lake was formed about 18,000 years ago, at the time the large alpine glaciers melted. It is fed by many small rivers from the surrounding mountains and a powerful underwater source, the Boubioz, at a 82-metre depth.