The Semnon (French pronunciation: [sɛmnɔ̃]; Breton: Sevnon) is a 73.3 km (45.5 mi) long river in the Mayenne, Maine-et-Loire, Ille-et-Vilaine and Loire-Atlantique départements, northwestern France.

It is a left tributary of the Vilaine into which it flows between Pléchâtel and Bourg-des-Comptes.

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