| Semnon | |
| Pushpin Map: | France#France Bretagne |
| Source1 Location: | Congrier |
| Source1 Coordinates: | 47.7917°N -1.1428°W |
| Mouth Coordinates: | 47.8997°N -1.7569°W |
| Subdivision Type1: | Country |
| Length: | 73.3km (45.5miles) |
| Source1 Elevation: | 90m (300feet) |
| Mouth Elevation: | 7m (23feet) |
| Discharge1 Avg: | 2.8m3/s |
| Basin Size: | 400km2 |
The Semnon (in French pronounced as /sɛmnɔ̃/; Breton: Sevnon) is a 73.3km (45.5miles) 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.
This list is ordered from source to mouth: