Tapajós River, Portuguese Rio Tapajós, central Brazil, formed by the union of the Teles Pires and the Juruena rivers, in north-central Mato Grosso estado (state). It winds northward through the Mato Grosso plateau and forms the state border between Mato Grosso and Amazonas and then between Pará and Amazonas states. It bends north-northeastward, traverses Pará, and empties into the Amazon River just above Santarém, after a course of about 400 miles (650 km). Its length, including its longest tributary, the Teles Pires, is 1,238 miles (1,992 km).

Although the Tapajós is interrupted by rapids, its entire length is navigable. It is even possible for rivercraft to ply the Tapajós and the Arinos, pass through a canal into the Cuiaba River, a tributary of the Paraguai Paraguay River, and eventually reach Buenos Aires, Arg. Settlement along the Tapajós is sparse, and many of its tributaries are uncharted; but there are several important rubber plantations on its banks.