Lago Ritom, reservoir in Ticino, Switzerland

Lago Ritom is a reservoir in the Piora Valley, a side valley of the Leventina in the municipality of Quinto, Ticino. It was formed by a glacial basin and enlarged for the Ritom hydroelectric plant of the Swiss Federal Railways. Its water is held by a gravity dam, 309 metres long and 23 metres high, built in 1950; the small Lago di Tom lies to the north.

The first dam at Lago Ritom was constructed in 1918 to generate hydroelectric power for the surrounding region. In 1950, the dam was raised by about 23 meters to store more water and increase electricity generation for the Gotthard railway line and local communities.

The name Ritom comes from the Ri di Tom stream flowing from Lago di Tom above, literally "lake of Tom's stream". The lake holds brown trout, lake trout, rainbow trout, Arctic char, brook trout and minnow. The funicular from Piotta, 1369 m long with a maximum gradient of 87.8%, is among the steepest in the world.

The lake is reached on foot via old mule tracks, by a paved road, or by the Piotta–Ritom funicular, which runs along the pressure pipe from 1,007 m in Piotta and climbs 1,369 m at a maximum gradient of 87.8%. A guesthouse stands by the dam. The dam crest is 309 m long and 23 m high, but the lake cannot be filled completely because the upper ground is porous.

Workers died during dam construction after drinking lake water, according to local accounts. This is cited as evidence that the original Lago Ritom, like nearby Lago di Cadagno today, was meromictic: its water layers stayed separate and did not mix seasonally.