Lake St Clair or leeawulenna is a natural freshwater lake located in the Central Highlands area of Tasmania, Australia. The lake forms the southern end of the Cradle Mountain-Lake St Clair National Park. It has an area of approximately 45 square kilometres (17 sq mi), and a maximum depth of 215 metres (705 ft), making it Australia's deepest lake.

Cradle Mountain–Lake St Clair National Park is located in the Central Highlands area of Tasmania (Australia), 165 kilometres (103 mi) northwest of Hobart. The park contains many walking trails, and is where hikes along the well-known Overland Track usually begin. Major features are Cradle Mountain and Barn Bluff in the northern end, Mount Pelion East, Mount Pelion West, Mount Oakleigh and Mount Ossa in the middle and Lake St Clair in the southern end of the park. The park is part of the Tasmanian Wilderness World Heritage Area.