Emblematic of the wetlands of South America, where it is the largest heron species, the cocoi heron is a distinctive waterbird with an all-black cap, a white neck, white plumes on the breast, a grey back and wings, and white thighs. The lower underparts are black, a few black streaks may run along the midline of the foreneck, and black ‘shoulder’ patches show on the wings when the bird is standing. During the breeding season, the cocoi heron has long, black plumes on the cap, tipped with white, a bright yellow beak with a reddish base, and dusky pink legs. The eyes are yellow and the facial skin is greenish to blue. Outside of the breeding season, the beak is a duller yellow, with a blackish base, and the legs are blackish. The male and female cocoi heron are similar in appearance, whereas immature cocoi herons are greyer, and have a duller but still distinctive dark cap, which lacks plumes.

The cocoi heron is similar in appearance to the closely related great blue heron (Ardea herodias), but is much whiter, and can also be distinguished by the completely black cap, the white rather than grey neck, and white rather than chestnut thighs and wing edges. Southern cocoi herons are reported to be larger than those in the north, and there is also likely to be further geographical variation that has not yet been described.