The Tapajós ( ) is a river in Brazil. It runs through the Amazon Rainforest and is a major tributary of the Amazon River. When combined with the Juruena River, the Tapajós is approximately long. For most of its length it runs through Pará State, but the upper (southern) part forms the border between Pará and Amazonas State. The source is at the Juruena–Teles Pires river junction. The Tapajós River basin accounts for 6% of the water in the Amazon Basin, making it the fifth largest in the system.