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.

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