Ostprignitz-Ruppin is a Kreis (district) in the northwestern part of Brandenburg, Germany. Neighboring are (from north clockwise) the districts Müritz and Mecklenburg-Strelitz in Mecklenburg-Western Pomerania, the districts Oberhavel and Havelland, the district Stendal in Saxony-Anhalt, and the district Prignitz. The district consists of plain countryside northwest of Berlin. It is densely forested. Many rivers and lakes can be found in its western, its central and its northeastern part. The main river in the district is the Dosse.