The Tamnava originates from several small streams from the hill of Konjsko brdo (Cyrillic: Коњско брдо; Horse hill) on the northern slopes of Vlašić mountain, northwest of the town of Valjevo, less than a kilometer away from the source of its major tributary, the Ub River.

At the village of Kamenica, the Tamnava makes almost complete circle (triangle, in this case) as it comes just a kilometer away from its own furthermost source.

In this early flow, the Tamnava makes the southeastern border of the Pocerina region of western Serbia.

The river turns northeast and after it passes the village of Liso Polje flows into the Kolubara, south of the Belgrade's suburb of Obrenovac.

The Tamnava drains an area of 930 km2 (360 sq mi), through the Kolubara belongs to the Black Sea drainage basin and it is not navigable.