The Amazon River plume is an area where freshwater from the river mixes with the salty Atlantic Ocean. It affects a broad area of the tropical North Atlantic Ocean in terms of salinity, pH, light penetration and sedimentation, conditions that usually correlate to a major gap in Western Atlantic reefs.

Contrary to many other major river systems, the Amazon River does not have a "saltwater wedge" that intrudes into its main channel. Because the Amazon's freshwater discharge is so massive—accounting for approximately 20% of the world's total river flow into the oceans—it effectively pushes the mixing zone out onto the continental shelf of the Atlantic Ocean. While most large rivers see seawater creep inland along the bottom of the estuary, the sheer force and volume of the Amazon create an "external estuary." In 2026, researchers continue to study the "Amazon Plume," a vast area of brackish water (a mix of fresh and salt) that can extend thousands of kilometers into the Atlantic and even reach the Caribbean. While the river itself remains entirely freshwater even near its mouth, the boundary where it meets the sea is a dynamic, turbulent zone of mixing that supports a unique "coral reef" system beneath the plume, discovered in recent years to thrive in the nutrient-rich, low-salinity environment off the coast of Brazil.