|Primary inflows||River Derwent, Watendlath Beck, Brockle Beck|
|Primary outflows||River Derwent|
|Max. length||4.6 kilometres (2.9 mi)|
|Max. width||1.91 kilometres (1.19 mi)|
|Surface area||2 sq mi (5.2 km2)|
|Average depth||5.5 m (18 ft)|
|Max. depth||22 m (72 ft)|
|Water volume||29 x 106 m³|
|Residence time||55 days|
|Shore length1||9.6 mi (15.4 km)|
|Surface elevation||75 metres (246 ft)|
|Islands||4 plus 9 small (13)|
|1 Shore length is not a well-defined measure.|
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 (4.8 km) long by 1 mile (1.6 km) wide and is some 72 feet (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.
Derwentwater is a place of considerable scenic value. It is surrounded by hills (known locally as fells), 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 an extensive network of footpaths exists within the hills and woods surrounding the lake.
The Keswick—Borrowdale road runs along the eastern shore of the lake and carries a regular bus service. There is a lesser, or unclassified, road along the western shore connecting the villages of Grange and Portinscale.
Derwentwater gave its name to the Earldom of Derwentwater.
The lake is believed to be the last remaining native habitat of the vendace (Coregonus vandesius) fish from the 4 originally known sites: Bassenthwaite Lake and Derwent Water in the Lake District and the Castle Loch & Mill Loch in Lochmaben.
'Derwent' is " '(River) with oak trees', traditionally explained from Brit." (i.e.: Brythonic Celtic) " 'derwā' 'oak' plus suffixes, hence of the same origin as other English rivers named Derwent, Darwen, Darent and Dart...The river gave its name to Derwent Water (which was also known as the 'Lake of Derwent', 'Keswick Lake', or 'Keswick Water' in the 18th-19th centuries)...". Plus "OE 'wæter', with the meaning probably influenced by its ON relative 'vatn'." 
There are numerous islands in Derwentwater, the largest being: Derwent Island, Lord's Island, St Herbert's Island, Rampsholme Island, Park Neb, Otter Island, and Otterbield Island. St. Herbert's Island is named after a C. 7th priest hermit, St. Herbert of Derwentwater.
- McNamara, Jane, Table of lake facts, Environment Agency of England and Wales, archived from the original on June 28, 2009, retrieved 2007-11-13
- "Ice Age fish thrives in new home". BBC News. 2008-02-27. Retrieved 2010-05-01.
- Whaley, Diana (2006). A dictionary of Lake District place-names. Nottingham: English Place-Name Society. pp. lx,423 p.95. ISBN 0904889726.
- Whaley, 2006, p.422
- Derwentwater in the snow, January 2010. Daily Mail article
- The Cumbria Directory - Derwent Water
- Lake District Walks - Derwent Water Keswick
- Media related to Derwent Water at Wikimedia Commons