The Rio Arapiuns is a blackwater river in Pará state, north-central Brazil, flowing as a tributary of the Rio Tapajós. It joins the Tapajós approximately 30 kilometers before the Tapajós itself merges with the Amazon River. The river's coordinates place it near 2°18′S, 55°00′W, in the greater Santarém region.

The confluence of the Rio Arapiuns and the Rio Tapajós is a notable feature for paddlers: depending on wave conditions, the meeting of the two rivers can resemble open-sea conditions, which warrants caution for those in smaller craft. The river's blackwater character gives it a distinctive dark appearance typical of tannin-rich Amazonian tributaries.

The banks of the Rio Arapiuns are lined with numerous beaches, among the most well-known being Praia do Icuxi, Praia da Ponta do Toronó, and Praia da Ponta Grande. The surrounding landscape is dense Amazonian rainforest, and the river corridor passes through or near several indigenous and traditional communities, including the village of Urucureá and the community of Carão. The region is home to multiple indigenous peoples and sits within a broader area of recognized indigenous territories.

Wildlife along the river includes alligators, iguanas, parrots, monkeys, sloths, otters, and turtles, particularly in adjacent lakes and channels such as Lago do Jacare and the Canal do Jari. The area around Ponta das Pedras and Lago Tapari also offers notable natural scenery. Giant water lilies are present in certain sections.

Organized tours operating out of Alter do Chão and Santarém access the river by motorized boat, typically as full-day or multi-day excursions. Kayaking and canoeing are offered as activities in the broader Alter do Chão area, which serves as a practical base for paddling trips on the Arapiuns. The river is accessible via the PA-257 road near Santarém. Community-based tourism is active along the river, with local residents engaged in agroextractive activities, and paddlers should be respectful of community lands and indigenous territories throughout the route.