The hoatzin (/hoʊˈætsɪn/ hoh-AT-sin) or hoactzin (/hoʊˈæktsɪn/ hoh-AKT-sin) (Opisthocomus hoazin) is a species of tropical bird found in swamps, riparian forests, and mangroves of the Amazon and Orinoco Basins in South America. It is the only extant species in the genus Opisthocomus which is the only extant genus in the Opisthocomidae family under the order of Opisthocomiformes. Despite being the subject of intense debate by specialists, the taxonomic position of this family is still far from clear.

The hoatzin is notable for its chicks having primitive claws on two of their wing digits. It is unique among birds in possessing a digestive system that significantly supports the fermentation and the effective breakdown of plant matter, a trait more commonly known from herbivorous ungulate-ruminant mammals and some primates. This bird is also the national bird of Guyana, where the local name for this bird is Canje pheasant.

The hoatzin is pheasant-sized, with a total length of 65 cm (26 in), and a long neck and small head. It has an unfeathered, blue face with maroon eyes, and its head is topped by a spiky, rufous crest. The long, sooty-brown tail is bronze-green tipped with a broad, whitish or buff band at the end. The upper parts are dark, sooty brown-edged buff on the wing coverts and streaked buff on the mantle and nape. The underparts are buff, while the crissum (the undertail coverts surrounding the cloaca), primaries, underwing coverts, and flanks are rich rufous-chestnut, but this is mainly visible when the hoatzin opens its wings.

It is a noisy bird, and makes a variety of hoarse calls, including groans, croaks, hisses, and grunts. These calls are often associated with body movements, such as wing spreading.

Hoatzin chicks have two claws on each wing. Immediately after hatching, they can use these claws and their oversized feet to scramble around tree branches without falling into the water. When predators such as the great black hawk attack a hoatzin nesting colony, the adults fly noisily about, trying to divert the predator's attention, while the chicks move away from the nest and hide among the thickets. If discovered, however, they drop into the water and swim under the surface to escape, then later use their clawed wings to climb back to the safety of the nest. This has led to comparisons to the fossil bird Archaeopteryx, but the characteristic is rather an autapomorphy, possibly caused by an atavism toward the dinosaurian finger claws, whose developmental genetics ("blueprint") is presumably still present in the avian genome. Furthermore, wing claws are not unique to hoatzins, with chickens also bearing them. Since Archaeopteryx had three functional claws on each wing, some earlier systematists speculated that the hoatzin was descended from it, because nestling hoatzins have two functional claws on each wing. Modern researchers, however, hypothesize that the young hoatzin's claws are of more recent origin, and may be a secondary adaptation from its frequent need to leave the nest and climb about in dense vines and trees well before it can fly. A similar trait is seen in turacos, whose nestlings use claws on their wings to climb in trees.

The generic name Opisthocomus comes from Ancient Greek ὄπισθοκομος ópisthokomos derived from ὄπισθε ópisthe (ὄπισθεν ópisthen before a consonant) "behind" and κόμη kómē "hair" altogether meaning "long hair behind" referring to its large crest.

The hoatzin was originally described in 1776 by German zoologist Statius Müller. Much debate has occurred about the hoatzin's relationships with other birds. Because of its distinctness, it has been given its own family, the Opisthocomidae, and its own order, the Opisthocomiformes. At various times, it has been allied with such taxa as the tinamous, the Galliformes (gamebirds), the rails, the bustards, seriemas, sandgrouse, doves, turacos and other Cuculiformes, and mousebirds. A whole genome sequencing study published in 2014 places the hoatzin as the sister taxon of a clade composed of Gruiformes (cranes) and Charadriiformes (plovers). Another genomic study in 2024 instead places it as the sister group to the Phaethoquornithes (containing numerous aquatic bird orders). The combined group was found to be sister to the Mirandornithes (flamingos and grebes).

In 2015, genetic research indicated that the hoatzin is the last surviving member of a bird line that branched off in its own direction 64 million years ago, shortly after the extinction event that killed the nonavian dinosaurs. Another genetic study from 2024 instead suggested a Late Cretaceous origin (around 70 million years ago), but found that this early divergence is shared with a majority of extant bird orders, making it no more basal than they are.