Lake Achen (German: Achensee) is a lake north of Jenbach in Tyrol, Austria. It is the largest lake within the federal state, and has a maximal depth of 133 metres. Together with the Achen Valley it parts the Karwendel mountain range in the west from the Brandenberg Alps in the east. Water quality is near drinking water, with sight up to 10 m below the surface. An alpine lake, water temperature is accordingly low, rarely above 20 °C. Its size and wind conditions make it suitable for windsurfing. Eben am Achensee, Achenkirch, and Pertisau are municipalities on the lake.