A view of Derwent Water in the English Lake District

Derwent Water (alternative spelling, Derwentwater) is one of the principal bodies of water in the Lake District National Park in the north of England.

The lake occupies part of Borrowdale and lays immediately south of the town of Keswick. It is both fed and drained by the River Derwent. It measures approximately 4km (3 miles) long by 1.5km (1 mile) wide and is some 22 metres deep. There are several islands within the lake, of which at least one is inhabited.

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 two lakeside marinas, one at Keswick and one at nearby Portinscale, 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 between the villages of Grange and Portinscale.