The Iskar (Bulgarian: Искър; Latin: Oescus; Ancient Greek: Οἶσκος) is, with a length of 368 km, the longest river that runs solely in Bulgaria, and a tributary of the Danube.

The Iskar is formed by three rivers, the Cherni (black) Iskar, Beli (white) Iskar and Levi (left) Iskar (known under the collective name Iskrove), with the source being accepted to be the Prav (straight) Iskar, a tributary of the Cherni Iskar. After descending from the north slopes of Rila, it fills the Iskar Reservoir—the largest in Bulgaria and the Lake Pancharevo. The river runs near Sofia and passes through a rocky gorge in the Balkan Mountains. It flows into the Danube near the village of Gigen in Pleven Province. The Iskar is the only river to take its source from southern Bulgaria and cross the Balkan Mountains to flow into the Danube.

The Iskar flows through seven provinces of Bulgaria: the capital city of Sofia, Sofia Province, Pernik Province, Vratsa Province, Pleven Province and Lovech Province. This is the main source of water for Sofiyskavoda, the water and sewarage company in Sofia.