Hammersø (English: Hammer Lake), or Hammer Sø, is the largest lake on the island of Bornholm and the only tarn in Denmark.

[1] It is located in a valley in Hammeren, the northernmost point of the island.

A narrow isthmus separates the lake from Opalsø, an artificial lake formed in a granite quarry which was closed in 1970.

Its encatchment area is approximately 133 hectares (330 acres), consisting of about 50% rocks, 25% forest and 25% cultivated land.

This article about a location in the Capital Region of Denmark is a stub.