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 lake's 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.