Þingvallavatn is a lake in south-western Iceland. With a surface of 84 km² it is the largest natural lake in Iceland. Its greatest depth is at 114 m. At the northern shore of the lake, at Þingvellir, the Alþingi, the national parliament, was founded in the year 930.
The lake is part of the Þingvellir National Park. The volcanic origin of the islands in the lake is clearly visible. The cracks and faults around it, of which the famous Almannagjá canyon 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 Sog River.