| Formoso River | |
|---|---|
| Native name | Rio Formoso | 
| Country | Brazil | 
| Basin features | |
| Main source | Rondônia state | 
| River mouth | 9°53′45″S 64°19′08″W / 9.895884°S 64.318890°WCoordinates: 9°53′45″S 64°19′08″W / 9.895884°S 64.318890°W | 
| River system | Jaci Paraná River | 
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.