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 3 miles long by 1 mile wide and is some 72 feet 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.
Derwentwater is a place of considerable scenic value. It is surrounded by hills, and many of the slopes facing Derwentwater are extensively wooded. A regular passenger launch operates on the lake, taking passengers between various landing stages. There are seven lakeside marinas, the most popular stops being Keswick, Portinscale and the Lodore Falls, from which boats may be hired. Recreational walking is a major tourist activity in the area and there is an extensive network of footpaths in the hills and woods surrounding the lake.