Lago Tremorgio, Mountain lake above Rodi-Fiesso, Quinto, Ticino, Switzerland

Lago Tremorgio is an alpine lake at 1,820 m above Rodi-Fiesso in the municipality of Quinto, Ticino. It lies in a hollow framed by Pizzo di Mezzodì, Poncione di Tremorgio, Pizzo Meda and Cima di Filo, in limestone Bündnerschiefer of the Mesozoic. Almost circular with earthy shores, it reaches a maximum depth of 50 m; a canal now drains it underground, and a trail crosses the former lake bed.

In 1899 the engineer Raffaele Frasa of Lavorgo applied for the industrial water rights of Lago Tremorgio. Construction of the hydroelectric plant began only in 1918, and the water was channelled to the Tremorgio power station at Rodi. The plant was completed in 1926 with pumps that raised water from the Ticino river into the lake; these pumps stopped operating in 1975.

The lake lies above Rodi-Fiesso, a village in the municipality of Quinto; the Italian Wikipedia lists it under Prato Leventina, reflecting a boundary change. The Capanna Tremorgio near the shore serves as a base for hikers heading over the Passo del Campolungo into Val Lavizzara. The lake is stocked with brown trout, rainbow trout, Arctic char and Canadian trout; Canadian trout stocking was suspended in 1983 and later resumed with a limited number of summer fish.

Visitors can reach the lake in about five minutes by cable car from Rodi-Fiesso, which departs from a parking area near the village, or hike up on marked footpaths from below. The walking trails around the lake are well-signed and suitable for different difficulty levels, with the quietest conditions early in the morning or late in the afternoon.

Although Lago Tremorgio is not a true storage reservoir, part of its water drops 850 metres to the Tremorgio power station at Rodi-Fiesso. A small dam was once built, but the ground proved unsuitable and heavy seepage losses made it unusable. The lake's surface has also been mapped lower over time: 1,830 m in the 1990s, 1,827 m in 2011 and 1,820 m on current federal maps.