Unterwasser (English: Underwater) is a village in the Toggenburg region of the canton of St. Gallen, Switzerland, situated at the confluence of the two streams forming the upper Thur (known as Säntisthur and Wildhausthur).
It is a relatively small ski resort, and many of its trails are shared with nearby Alt st. Johan.
Iltiosbahn, a funicular leads to Iltios and the aerial cableway to Chäserrugg.
The toponym was first recorded in the 15th century as under dem Wasser , or "below the water".
Ski jumper Simon Ammann — winner of four gold medals in the 2002 and 2010 Winter Olympics, and World Champion 2007 — comes from Unterwasser.