thumb|right|The Clain rivers in [[Chasseneuil-du-Poitou]]
The Clain (; ) is a long river in western France, a left tributary of the river Vienne. Its source is near Hiesse, Charente.
The Clain flows generally north, through the following departments and towns:
- Charente
- Vienne: Pressac, Vivonne, Poitiers.
It joins the Vienne near Châtellerault, Cenon-sur-Vienne.
Among its tributaries are the Boivre, the Clouère and the Vonne.