Lake Sils (German Silsersee, Romansh: Lej da Segl) is a lake in the Upper Engadine valley, Grisons, Switzerland. It takes its name from the village of Sils im Engadin.
The lake lies at an altitude of 1,797 metres above sea level, between Maloja Pass and Lake Silvaplana. Several mountains over 3,000 metres overlook the lake, notably Piz Corvatsch, Piz Grevasalvas and Piz da la Margna. The lakes drainage basin culminates at Piz Fora (3,336 m) and comprises the Vadrec da Fedoz glacier.
With a surface area of 4.1 km2, Lake Sils is the largest lake of the Engadine. It is also the largest natural lake in the Alps lying above 1,000 metres. The lake was formed by a prehistoric landslide dam, similarly to Lake Silvaplana.
