Rio Negro, a river and important tributary of the Amazon River in South America. It begins at the junction of the Guainía and Casiquiare rivers on the Colombia-Venezuela boundary and forms the boundary between those two countries as it flows southward into Brazil. The river then flows southeastward to the Amazon at Manaus, head of deepwater navigation on the Amazon. The Rio Negro is linked to the Orinoco River, the chief river of northern South America, by the Casiquiare.

The Rio Negro is about 1,240 miles (2,000 km) long and drains an area of 386,000 square miles (1,000,000 km2). The flow of the river is more than twice that of the Mississippi. The lower course of the Rio Negro is as much as 20 miles (32 km) wide and contains numerous islands and channels. The Branco is the chief tributary.

The Rio Negro was discovered in 1638 by Pidro Teixeira, a Portuguese explorer. The river's name means "black river" and comes from the dark color of the water.