The hoatzin or hoactzin is a species of tropical bird found in swamps, riparian forests, and mangroves of the Amazon and the 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.

Opisthocomidae is a family of birds, the only named family within the order Opisthocomiformes. The only living representative is the hoatzin which lives in the Amazon and the Orinoco delta in South America. Several fossil species have been identified, including one from Africa and one from Europe.