The Feldsee (also Feldbergsee) is a lake in southern Baden-Württemberg at the foot of the Feldberg east of Freiburg im Breisgau in Germany.

[1] The largest tarn in the Black Forest, it is hemmed in on three sides by steep mountainsides up to 300 metres high.

This area of highland at a height of 1,100 m, which is open to the northeast, enabled it to amass and retain the huge quantities of snow that were the cause of this armchair-shaped terrain with its steep back face, level floor and embankment of moraine at the front.

Below the Feldsee, between the heaps of moraine, used to be another, smaller lake that, through the formation of peat[2] has silted up to become the present-day, botanically valuable bog of Feldseemoor.

This is in order to protect a rare underwater fern, the spiny quillwort (Isoetes echinospora), that thrives in water 1–2 metres deep and only occurs in Germany in the Feldsee and Lake Titisee.