Derwentwater, or Derwent Water, is a lake in the Lake District in North West England, immediately south of Keswick. It is in the unitary authority of Cumberland within the ceremonial county of Cumbria. It is the third largest lake by area, after Windermere and Ullswater. It has a length of 4.6 kilometres (2.9 mi), a maximum width of 1.91 kilometres (1.19 mi), and an area of 5.4 square kilometres (2.1 sq mi). Its primary inflow and outflow is the River Derwent, which also flows through Bassenthwaite Lake before reaching the Irish Sea at Workington. There are several islands within the lake, one of which is inhabited. 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.

The Lake District, also known as the Lakes or Lakeland, is a mountainous region and national park in Cumbria, North West England. It is famous for its landscape, including its lakes, coast, and the Cumbrian mountains; and for its literary associations with Beatrix Potter, John Ruskin, and the Lake Poets.