Þingvallavatn is a rift valley lake in southwestern Iceland. With a surface of 84 km² it is the largest natural lake in Iceland. Its greatest depth is 114 m. At the northern shore of the lake, at Þingvellir (after which the lake is named), the Alþingi, the national parliament, was founded in the year 930, and held its sessions there until 1799.
Surrounding the rift valley we can for example see the mountains Skjaldbreiður, Botnssúlur, Búrfell Í Grímsnesi, Hengill and further along towards the horizon Vífilsfell, Esja Mountain, Iceland and Skálafell – Skíðasvæði.
The lake lies partially within Thingvellier National Park, Iceland. The volcanic origin of the islands in the lake is clearly visible. The cracks and faults around it, of which the Almannagjá ravine is the largest, is where the Eurasian and North American tectonic plates meet. Silfra fissure is a popular scuba and snorkeling site. The only outflow from lake Þingvallavatn is the river Sog. One of the noted features of the lake is the presence of four morphs of the Arctic char.