Osogovo is the northernmost mountain in the Osogovo-Belassitsa mountain range, as well as the highest mountain in this range. Across its main ridge, from southeast to northwest, the mountain is divided between Bulgaria and the Republic of North Macedonia, with the smaller northeastern portion lying within Bulgarian territory and the larger southwestern portion within North Macedonian territory.
From southwest to northeast, the mountain range extends 67–68 km in length and 32–34 km in width. Its total area is 4,223 square kilometers, of which 996 square kilometers lie between the Kyustendil and Kamenitsa basins.
The part of Osogovo located in Bulgaria is divided into two sections: the western (or high) section and the eastern (or low) section. The two sections of the Osogovo Mountains differ significantly in terms of structure, rock age, and relief. The Novoselska River and the Dobra River are considered the boundary between the two sections.
Osogovo has an asymmetrical profile—a steep northern slope along the length of the Kyustendil Fault and a gentle southeastern slope toward the valley of the Eleshnitsa River.
Overall, the mountain is composed of crystalline rocks, within which a massive granite pluton is embedded. It is cut through by Tertiary volcanic bodies, and its southeastern slopes are covered by sedimentary rocks.
Osogovo is an important hydrographic hub. The river network has a high density of over 2 kilometers per square kilometer. The rivers that originate in the Bulgarian part of the mountain are right-bank tributaries of the Struma River; the most significant of these are the Bistritsa, Eleshnitsa, Dragovishitsa, and Treklyanska Rivers. The Bistritsa and Eleshnitsa Rivers originate immediately below Mount Ruen. The Osogovo Mountains have a runoff rate of approximately 20 liters per second per square kilometer. In the valleys of the Eleshnitsa and Rechitsa Rivers, there are karst springs.
The “Tsarna Reka” Nature Reserve is located within the mountain; it is the only protected area in the range, covering a total area of 190.8 hectares, of which 185.3 hectares are forested. It was established to protect characteristic forest ecosystems. Human activities are not permitted within the reserve.
In the Osogovo Mountains, you can see giant sequoia trees in the “Yuch Bunar” area within the territory of the village of Bogoslov, as well as the “Centuries-Old Beech Forest” in the “Sveta Gora” area—the “St. Luke” Monastery within the territory of the village of Granitsa.
