Melanosuchus niger sits within Alligatoridae alongside the alligators and the other caimans. Large males reach and occasionally exceed four metres, making it the largest living alligatorid and the apex predator of Amazonian rivers and flooded forest. Its diet spans fish, including piranha and large catfish, along with capybara, birds, turtles and smaller caimans.
The dark pigmentation that gives the species its name is thought to assist both heat absorption and night-time concealment. Juveniles carry pale yellow or white banding on the flanks that fades with age. The eyes are large relative to those of other caimans, consistent with heavily nocturnal foraging.
Commercial hide hunting from the 1940s to the 1970s removed a large majority of the population, and the species was once counted among the most depleted crocodilians. It has recovered sufficiently to be assessed as Least Concern globally, though recovery is uneven and populations near dense settlement remain reduced. Reported lengths above five metres circulate widely but are poorly documented.