The Rio Santo Antônio is a river in Bahia state, eastern Brazil, flowing through the Chapada Diamantina region as a tributary of the Paraguaçu River. Its mouth is located at approximately 12°46′S, 41°16′W. Together with the Rio Utinga, it feeds the Marimbus wetlands, a large swamp and semi-wetland area where the two rivers converge.

The Marimbus area formed by the Rio Santo Antônio and Rio Utinga is the primary draw for paddlers in this region. Canoe trips through the Marimbus marsh are characterized by calm, navigable waters that wind through mazes of water lilies, with the Serra do Sincora mountains providing the surrounding backdrop. Organized canoe tours of approximately 1.5 hours operate through this wetland zone. The varied flora and fauna of the convergence area are notable features, with birds and Capuchin monkeys commonly observed along the route. Capybaras and caimans are also present in the area, though sightings are less frequent.

Given the presence of caimans in the wetlands, paddlers should be aware of this wildlife when on the water. The Marimbus region is part of the broader Chapada Diamantina ecosystem, and the calm water character of the marsh makes it accessible for canoe-based exploration rather than technical whitewater paddling.