Temporal range: Late Pleistocene - Present
|Adult above, immature below|
| Melanosuchus niger|
The black caiman has dark-coloured, scaly skin. The skin coloration helps with camouflage during its nocturnal hunts, but may also help absorb heat (see thermoregulation). The lower jaw has grey banding (brown in older animals), and pale yellow or white bands are present across the flanks of the body, although these are much more prominent in juveniles. This banding fades only gradually as the animal matures. The bony ridge extending from above the eyes down the snout, as seen in other caiman, is present. The eyes are large, as befits its largely nocturnal activity, and brown in colour. Mothers on guard near their nests are tormented by blood-sucking flies that gather around their vulnerable eyes leaving them bloodshot.
The black caiman is structurally dissimilar to other caiman species, particularly in the shape of the skull. Compared to other caimans, it has distinctly larger eyes. Although the snout is relatively narrow, the skull (given the species' considerably larger size) is much larger overall than other caimans. Black caiman are relatively more robust than other crocodilians of comparable length. For example, a 3.9 m (13 ft) adult was found to have a considerably heavier and longer skull than a 4.8 m (16 ft) Nile crocodile (Crocodylus niloticus). Young black caiman can be distinguished from large spectacled caiman by their proportionately larger head, as well as by the colour of the jaw, which is light coloured in the Spectacled caiman and dark with three black spots in the black caiman.
The black caiman is one of the largest extant reptiles. It is the largest predator in the Amazon basin and possibly the largest member of the family Alligatoridae. Most adult black caimans are 2.8 to 4.26 metres (9.2–14 ft) in length, with a few old males growing larger than 5 m (16 ft) and exceeding a weight of 400 kg (880 lb). Sub-adult male specimens of around 2.5–3.35 m (8.2–11.0 ft) will weigh roughly 95–100 kg (209–220 lb), around the same size as a mature female, but will quickly increase in bulk and weight. Mid-sized mature males of 3.5–4 m (11–13 ft) weigh approximately 300 kg (660 lb). Another sampling of subadult males found them to range in length from 2.1 to 2.8 m (6.9 to 9.2 ft), averaging 2.45 m (8.0 ft), and that they weighed from 26 to 86 kg (57 to 190 lb), averaging 48 kg (106 lb). The black caiman broadly overlaps in size with the American alligator (Alligator mississippiensis), although it is on average larger at maturity. In some areas (such as the Araguaia River) this species is consistently reported at 4 to 5 metres (13–16 ft) in length, much larger than the alligator (which rarely even reaches 4 meters), although specimens this size are uncommon. Several widely reported but unconfirmed (and probably largely anecdotal) sources report that the black caiman can grow to over 6 m (20 ft) in length and weigh up to 1,100 kg (2,400 lb). In South America, two other crocodilians reportedly reach similar sizes: the American Crocodile (Crocodylus acutus) and the Orinoco Crocodile (C. intermedius).