The Cautário River (Portuguese: Rio Cautário) is a river of Rondônia state in western Brazil. It is a right tributary of the Guaporé River.

The Cautário River rises in the Uru-Eu-Wau-Wau Indigenous Territory. It is fed by streams from the 650 metres (2,130 ft) Serra Uopianes and the 750 metres (2,460 ft) Serra Pacaás Novos. The river runs in a southwest direction, forming the boundary between the Rio Cautário Federal Extractive Reserve and the Rio Cautário State Extractive Reserve. It flows into the Guaporé/Mamoré river, which in turn feeds the Madeira River at the city of Nova Mamoré.

The Cautário River has clear waters fed by a region without major deforestation and silting of the river's margins. It has rapids, but always with a drop of less than 2 metres (6 ft 7 in). These include Bom Destino, Desengano, Esperança and Cujubim. The most rugged stretch is between Redenção e Bom Destino. It is navigable, even in the middle section around Bom Destino, but only in the rainy season. During the dry season the river bed has extensive sandbanks.

• List of rivers of Rondônia