Rheas, also known as ñandus or South American ostrich, are moderately-sized South American ratites of the order Rheiformes. They are distantly related to the African ostriches and Australia's emu, with rheas placing just behind the emu in height and overall size.

A ratite is any of a group of mostly flightless birds within the infraclass Palaeognathae. They are mostly large, long-necked, and long-legged, the exception being the kiwi, which is also the only nocturnal extant ratite.