The Kolubara (Serbian Cyrillic: Колубара, pronounced [kɔlǔbara]), is the 123 km-long river in western Serbia; it is the eastern tributary to the Sava river.

The Kolubara originates as the Obnica, in the Podgorina region of the western Serbia, on the Povlen mountain under the Medvednik peak. It flows to the north, next to the village of Bobova, bends to the west (in this part the river is also known as Jadar), and at the city of Valjevo, it meets the river Jablanica from the south and forms the Kolubara, as the river is known for the remaining course. The Obnica is 25 km long. The Jablanica also originates from the Povlen mountain, just few kilometers away from the Obnica, under the Jablanik peak. It curves around the Parač mountain and next to the village of Balinović, before it meets the Obnica at Valjevo. The Jablanica is 24 km long. This is also the beginning of the 90 km long region of the Kolubara valley, divided in two large parts, referred to as Upper (gornja) Kolubara and Lower (donja) Kolubara (around the Belgrade's suburb of Obrenovac).