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.

[1] The park has two support bases on the banks of the Formoso River that are used by researchers and inspection teams.

This article related to a river in Rondônia, Brazil is a stub.