The black eagle is a large but slender eagle, at about 75 cm (30 in) in length and 148 to 182 cm (4 ft 10 in to 6 ft 0 in) in wingspan. Despite its large appearance (it is one of the largest eagles in its range), known weights are relatively modest, at between 1,000 and 1,600 g (2.2 and 3.5 lb), about half the weight of the partially sympatric mountain hawk-eagle, the latter being of a similar total length. Adults have all-black plumage, with a yellow bill base (cere) and feet. The wings are long and pinched in at the innermost primaries giving a distinctive shape. The tail shows faint barring and upper tail covers are paler. When perched the wing tips reach till or exceed the tail tip. The wings are held in a shallow V (wings just above the horizontal plane) in flight. Seen on hot afternoons, scouring the treetops for a nest to maraud, this bird is easily spotted by its jet black colour, large size, and a characteristic slow flight, sometimes just above the canopy.

Sexes are similar, but young birds have a buff head, underparts and underwing coverts. The wing shape helps to distinguish this species from the dark form of changeable hawk-eagle, (Nisaetus cirrhatus). The tarsi are fully feathered and the toes are relatively stout and short with long claws (particularly on the inner toe) that are less strongly curved than in other birds of prey.