The Semnon 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. Its source is in Congrier. It flows generally west-northwest. It is a left tributary of the Vilaine into which it flows between Pléchâtel and Bourg-des-Comptes.