The Zasavica (Serbian Cyrillic: Засавица, pronounced [ˈzâsaʋit͜sa]) is a river in the Mačva region in west-central Serbia.

It originates from several streams out of the swamps north of the village of Salaš Crnobarski, in the floodplain of the lower course of the Drina River.

At village of Banovo Polje, two major headstreams, the Jovača and Prekopac, meet, and from that point the river is called the Zasavica.

The final section is channeled (Bogaz canal), and the river often floods the surrounding area.

It actually flows through the typical elongated "mrtvaja" (oxbow), the old (fossil) bed of both the Sava and later, the Drina rivers.