Lake Millstatt (German: Millstätter See, sometimes written "Millstättersee", Slovene:Miljsko jezero) is a lake in Carinthia, Austria.
Lake Millstatt is situated in the Central Eastern Alps, near the town of Spittal an der Drau. With a surface area of 13.3 km2 (5.1 sq mi) it is the second largest lake of Carinthia (after Lake Wörth), though with 142 m (466 ft) by far the deepest and most voluminous. Its steep shore gives the lake a fjord-like character. The mountains in the north are up up to 2,100 metres high and protect it from cold winds, so the water temperature at the surface can reach 25 °C (77 °F) in summer. The narrow and long basin stretches over 12 km (7.5 mi) from east to west. In the south the wooded Hochgosch plateau with a breadth of 2 km (1.2 mi) and an elevation of 876 m (2,874 ft) divides the lake from the parallel Drava valley.
The name derives from the market town Millstatt at the northern shore. Other municipalities near the lake are Seeboden and Radenthein-Döbriach. The shaded southern shore, a protected area since 1970, is almost uninhabited. Since 1901 ship transport in summer is provided by the Millstättersee Shipping Company, which today runs four vessels.