The primary river that runs through the Amazon Rainforest is the Amazon River. It is the second-longest river in the world, measuring about 4,345 miles (7,062 kilometers). The river plays a crucial role in the ecosystem by supporting diverse wildlife and providing essential resources for local communities. It flows through several countries, including Brazil, Peru, and Colombia, and is characterized by numerous tributaries, making it a vital part of one of the most biodiverse regions on the planet.