In the waters The biggest water-dwelling mammal, and probably the biggest mammal in the Amazon altogether, is the Amazonian Manatee. A distant relative of the elephant, the Amazonian Manatee can grow up to 2.8m and weigh up to 540kg, with the female usually larger than the male of the species.

The title of the "biggest" animal in the Amazon River depends on whether you are looking for mass or length. The Amazonian Manatee is the largest mammal by weight, reaching up to 540kg (1,200 lbs) and nearly 3 meters in length. However, the Green Anaconda is arguably the most famous giant of the river, capable of reaching lengths of over 6 meters (20 feet) and weighing up to 250kg. If we consider fish, the Arapaima (or Pirarucu) is a prehistoric-looking giant that can grow up to 3 meters long and weigh 200kg. Finally, the Black Caiman is the river's apex predator, sometimes reaching lengths of 5 to 6 meters, making it a formidable contender for the largest overall creature. In 2026, conservation efforts continue to protect these giants from habitat loss and overfishing in the complex Amazonian ecosystem.