Lake Toma (German: 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 the canton of Grisons (Graubünden), Switzerland. Its surface area is 2.5 ha (6.2 acres).

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 46.499°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.