Þingvallavatn, anglicised as Thingvallavatn, is a rift valley lake in southwestern Iceland. With a surface of 84 square kilometers it is the largest natural lake in Iceland. Its greatest depth is 114 meters. At the northern shore of the lake, at Þingvellir, the Alþingi, the national parliament, was founded in the year 930, and held its sessions there until 1799 and still as of today the name Alþingi Íslendinga is carried by the parliament of Iceland.
The lake lies partially within Þingvellir National Park. 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.