The Oust (French pronunciation: [ust]; Breton: Oud) is a river in Brittany, France, right tributary of the Vilaine. It is 145 km (90 mi) long. Its source is in the hills between Corlay and Quintin. It flows generally southeast, through the following départements and towns:

The Oust flows into the river Vilaine in Redon. The part of the Oust between Rohan and Redon has been made navigable for small ships, and forms part of the Canal de Nantes à Brest.

The rivers Aff, Arz and Ninian are among its tributaries.