Derwentwater (or Derwent Water) is one of the principal bodies of water in the Lake District National Park in north west England. It lies wholly within the Borough of Allerdale, in the county of Cumbria.

The lake occupies part of Borrowdale and lies immediately south of the town of Keswick. It is both fed and drained by the River Derwent. It measures approximately is 4.8 km long by 1.6 km wide and is some 22 m deep. There are several islands within the lake, one of which is inhabited. Derwent Island House, an 18th-century residence, is a tenanted National Trust property open to the public on five days each year.