Ertholmene, generally called Christiansø, is a small archipelago situated approximately 18 km northeast of Bornholm in the Baltic Sea. Its permanent population is 92 (as of 2013)[citation needed] and its area is 39 hectares (0.39 km2). Ertholmene belongs to Denmark and has the country's easternmost point.

Ertholmene consists of three main islands, Christiansø (named after King Christian V), Frederiksø (named after King Frederick IV) and Græsholm, plus a number of minor rocks and skerries. The best known of the latter are Tat and Østerskær, Denmark's easternmost point. Christiansø makes up 22.3 hectares, Frederiksø 4, and Græsholm 11 hectares. Only Christiansø and Frederiksø are inhabited, while Græsholm is a bird reserve. The sound between Christiansø and Frederiksø, a well-sheltered natural harbour, is crossed by a pedestrian bridge that is pulled aside to accommodate larger vessels.