The Clain is a 144 km long river in western France, left tributary of the river Vienne. Its source is near Hiesse, in the Charente department.
The Clain flows generally north through the following departments and towns:
- Vienne: Pressac, Vivonne, Poitiers.
It joins the Vienne near Châtellerault, in Cenon-sur-Vienne.
Among its tributaries are the Boivre, the Clouère and the Vonne.