Seathwaite Tarn is a reservoir in the Furness Fells within the English Lake District. It is located to the south of Grey Friar and to the west of Brim Fell (on the ridge between The Old Man of Coniston and Swirl How) and north east of the village of Seathwaite on the east of the Duddon Valley.
In order to create a source of drinking water the existing tarn was considerably enlarged with a dam in 1904. During the dam construction some of the navvies rioted damaging buildings in the village, several rioters were shot, one dying the next day.
The dam is almost long and is concrete cored with slate buttresses, the resulting depth of the tarn being around . Water is not abstracted directly from the tarn, but flows some distance downriver to an off-take weir.
On the slopes of Brim Fell, above the head of the reservoir, are the remains of Seathwaite Tarn Mine. This was worked for copper in the mid 19th century, and also appears as a location in the novel The Plague Dogs by Richard Adams.