| Formoso River | |
| Pushpin Map: | Brazil |
| River System: | Jaci Paraná River |
| Source1 Location: | Rondônia state |
| Mouth Coordinates: | -9.8959°N -64.3189°W |
| Subdivision Type1: | Country |
| Subdivision Name1: | Brazil |
The Formoso River (Portuguese: Rio Formoso) is a river of Rondônia state in western Brazil.It is a tributary of the Jaci Paraná River.
In its upper reaches the Formoso River defines the western boundary of the Guajará-Mirim State Park.The park has two support bases on the banks of the Formoso River that are used by researchers and inspection teams.