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

Départements and communes along its course

This list is ordered from source to mouth:

• Mayenne: Congrier, Saint-Erblon, Senonnes
• Maine-et-Loire: Pouancé
• Ille-et-Vilaine: Eancé, Martigné-Ferchaud
• Loire-Atlantique: Fercé
• Ille-et-Vilaine: Thourie
• Loire-Atlantique: Soulvache
• Ille-et-Vilaine: Teillay, Ercé-en-Lamée, Lalleu, Tresbœuf, La Bosse-de-Bretagne, Bain-de-Bretagne, Pancé, Pléchâtel, Poligné, Bourg-des-Comptes