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Bassenthwaite Lake is one of the largest water bodies in the Lake District. It is long and narrow, approximately 4 miles long and 3⁄4-mile wide; it is also quite shallow, with a maximum depth of about 70 ft. The lake lies at the foot of Skiddaw, near the town of Keswick; it is fed by, and also drains into, the River Derwent..

Like the other Lake District lakes, Bassenthwaite Lake lies in a glacially eroded valley, left after the last glaciation. Bassenthwaite Lake is linked to Derwent Water by the River Derwent, which crosses the 3-mile alluvial plain between the two lakes. There has been speculation that Derwent Water and Bassenthwaite Lake were once one larger lake with the alluvial flats now separating them formed from partial infill of the original basin.

Bassenthwaite is the only body of water in the Lake District to be technically defined as a "lake" and to use the word "lake" in its name, all the others being "waters" (eg, Derwent Water), "meres" (eg, Windermere) or "tarns" (eg, Dock Tarn).