The black swan (Cygnus atratus) is a large waterbird, a species of swan which breeds mainly in the southeast and southwest regions of Australia. Within Australia, the black swan is nomadic, with erratic migration patterns dependent on climatic conditions. It is a large bird with black plumage and a red bill. It is a monogamous breeder, with both partners sharing incubation and cygnet-rearing duties.

Black swans can be found singly, or in loose companies numbering into the hundreds or even thousands. It is a popular bird in zoological gardens and bird collections, and escapees are sometimes seen outside their natural range. The black swan was introduced to various countries as an ornamental bird in the 1800s, but has managed to escape and form stable populations.

This bird is a regional symbol of both Western Australia, where it is native, and the English town of Dawlish, where it is an introduced species.

Described scientifically by English naturalist John Latham in 1790 as Anas atrata, the black swan was later placed into the genus Chenopis. In 1997, the species was placed in the genus Cygnus, in the subgenus Chenopis. The species is monotypic.

The black swan is common in the wetlands of southwestern and eastern Australia and adjacent coastal islands. In the south west its range encompasses an area between North West Cape, Cape Leeuwin and Eucla; while in the east it covers a large region bounded by the Atherton Tableland, the Eyre Peninsula and Tasmania, with the Murray–Darling basin supporting very large populations of black swans. It is uncommon in central and northern Australia.

The black swan's preferred habitat extends across fresh, brackish and salt water lakes, swamps and rivers with underwater and emergent vegetation for food and nesting materials. It also favours permanent wetlands, including ornamental lakes, but can also be found in flooded pastures and tidal mudflats, and occasionally on the open sea near islands or the shore.

The black swan was once thought to be sedentary, but is now known to be highly nomadic. There is no set migratory pattern, but rather opportunistic responses to either rainfall or drought. In high rainfall years, emigration occurs from the south west and south east into the interior, with a reverse migration from these heartlands in drier years. When rain does fall in the arid central regions, black swans will migrate to these areas to nest and raise their young. However, should dry conditions return before the young have been raised, the adult birds will abandon the nests and their eggs or cygnets and return to wetter areas.

The black swan, like many other water fowl, loses all its flight feathers at once when it moults after breeding and is unable to fly for about a month. During this time it will usually settle on large, open waters for safety.