Siljan, in Dalecarlia in central Sweden, is Sweden's sixth largest lake. The cumulative area of Siljan and the adjacent, smaller lakes Orsasjön and Insjön is 354 km². Siljan reaches a maximum depth of 120 m, and its surface is situated 161 metres above sea level. The largest city bordered by its shore is Mora.

The lake is located around the southwestern part of the perimeter of Siljansringen, a geological formation which was formed 365 million years ago by a major meteorite impact. The Cambrian-Silurian sedimentary layers in the area contain rich quantities of fossils. Some people suspect that there might be oil in the area, but drilling has been unsuccessful so far.