The Mostonga (Serbian Cyrillic: Мостонга) is a river in northern Serbia, in Bačka region of the province of Vojvodina, a 70-km long left tributary to the Danube.

The Mostonga originates from the marshy bogs north of the town of Sombor in northwestern Vojvodina, near the Nenadići hamlet.

It flows south (parallel to the Danube, the general direction of its whole course) and passes through the western outskirts of Sombor after which it is channeled for the first time, as a part of the Great Bačka Canal.

In this section the river makes a large bend (which begins already at Srpski Miletić and ends at Karavukovo), on a half of which it separates from the DTD.

The river enters the marshy terrain after the village of Mladenovo, west of the town of Bačka Palanka, before it empties into the Danube in one of its large meanders surrounding the Šarengradska ada island.