The Tomasee (Romansh: Lai da Tuma or Lag da Toma) is a lake at the northern face of Piz Badus, above the village of Tschamut in Grisons, Switzerland. Its surface area is 2.5 ha (6.2 acres).

Tomasee
Lai da Tuma
Tomasee Show map of Canton of Graubünden Tomasee Show map of Canton of Uri Tomasee Show map of Switzerland Tomasee Show map of Alps
Location	Grisons
Coordinates	46°37′57″N 8°40′20″E / 46.63250°N 8.67222°E / 46.63250; 8.67222
Primary inflows	Rein da Tuma
Primary outflows	Rein da Tuma
Basin countries	Switzerland
Surface area	2.5 ha (6.2 acres)
Surface elevation	2,345 m (7,694 ft)

It is the source of the Anterior Rhine and is deemed to be the official source of the Rhine (the source of the Posterior Rhine is above Hinterrhein, Switzerland, at 46°29′56″N 9°03′54″E / 46.499°N 9.065°E / 46.499; 9.065 (Source of Hinterrhein)).

It is possible to reach the lake on a path from Oberalp Pass, suitable for most walkers although still a mountain trail.^[1]