Lake Sirio, Glacial lake in Piedmont, Italy.

Lake Sirio is a glacial lake between Ivrea and Chiaverano in Piedmont, Italy. It covers about 0.3 km² and reaches a depth of nearly 45 metres, making it the largest of the five Laghi di Ivrea. A lakeside promenade with benches and picnic areas runs along the eastern-southwestern shore, where Montalto Dora Castle can be seen; the opposite bank is inaccessible because of steep terrain and houses.

The lake was formed by glaciers during the ice age in northern Italy. A significant milestone came in 1887 when the Società Canottieri Sirio rowing club was established on the southern shore, becoming one of the region's oldest rowing organizations.

The lake forms part of the Laghi di Ivrea system, a group of glacial lakes that shapes the identity of northern Piedmont. Locals and visitors use the shores for recreation and value it as a central element of their landscape.

The shoreline is easily accessible with several entry points for swimmers and water sports enthusiasts. Well-maintained paths around the lake allow visitors to explore at their own pace from different starting locations.

Lake Sirio is the only one of the five Laghi di Ivrea fed by a spring rather than by surface inflows. It lies within the Site of Community Importance "Laghi di Ivrea" (code IT1110021), established under the Habitats Directive and also designated a Special Area of Conservation.