Wast Water or Wastwater is a lake located in Wasdale, a valley in the western part of the Lake District National Park, England. The lake is almost 3 miles (4.8 km) long and more than one-third mile (500 m) wide. It is a glacial lake, formed in a glacially 'over-deepened' valley. It is the deepest lake in England] at 258 feet (79 m). The surface of the lake is about 200 feet (60 m) above sea level, while its bottom is over 50 feet (15 m) below sea level. It is owned by the National Trust.

The head of the Wasdale Valley, Wasdale Head, is a scattered agricultural hamlet. It is surrounded by some of the highest mountains in England, including Great Gable 899 m (2949 ft).