Þórisvatn (Icelandic pronunciation: [ˈθouːrɪsˌvahtn̥] ; sometimes anglicized to Thorisvatn) is the largest lake of Iceland, situated at the south end of the Sprengisandur plateau within the highlands of Iceland.

It is a reservoir with a surface area of about 88 km (34 sq mi) and is fed by the river Þjórsá, which comes down from the glacier Hofsjökull. On its southern end, Þórisvatn is used to generate electricity at a power station. It is like other Icelandic lakes, which are mostly glacial lakes or volcanic lakes.

The lake grew from about 70 km to 86 km with the construction of the power station and was previously only the second largest lake in the country.

• List of lakes of Iceland
• Geography of Iceland

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