Þórisvatn, (sometimes anglicized to Thorisvatn) is the largest lake of Iceland, situated at the south end of Sprengisandur highland road within the highlands of Iceland.

It is a reservoir of a surface about 88 km² and uses the energy of the river Þjórsá, which comes down from the glacier Hofsjökull. Here in the south, it is exploited in a power station. It is like other Icelandic lakes, which are mostly glacial lakes or volcanic lakes, it normally has a stark green color.