This relatively small eagle has a total length of 46 to 55 cm (18 to 22 in). It usually weighs 685 to 1,045 g (1 lb 8.2 oz to 2 lb 4.9 oz); but may exceed 1,262 g (2 lb 12.5 oz). The female is larger and heavier than the male. In one study, the wingspan of two individuals averaged 124 cm (49 in). Ayres' hawk-eagle has a long, barred tail and relatively narrow wings. The adult plumage is predominantly black/brown, with heavily streaked white underparts; in some individuals, the underparts are much paler.

Adults typically have a pronounced white shoulder where the leading edge of the wing meets the body (the carpal joint). The immature is drab and paler. The Ayres' hawk-eagle hunts small mammals like rabbits and mice.