The black-necked swan (Cygnus melancoryphus) is a species of waterfowl in the tribe Cygnini of the subfamily Anserinae.

[5] The black-necked swan is the only member of its genus that breeds in the neotropics and is the largest waterfowl native to South America.

It nests from Tierra del Fuego north to central Chile, Uruguay, and Rio Grande do Sul in extreme southern Brazil.

It withdraws from the southern half of Argentina in winter and is then found as far north as Brazil's São Paulo state.

[4] The black-necked swan inhabits freshwater marshes and swamps, shallow lakes, brackish lagoons, and sheltered coastal sites.

[4] The wetlands created in Chile by the 1960 Valdivia earthquake, such as Carlos Anwandter Nature Sanctuary on the Cruces River, have become important population centers for the black-necked swan.

Its nest is a mound of vegetation constructed by both members of a pair on a small islet or partially floating in a reedbed.

During that time, both sexes give a "soft, musical 'Whee-whee-whee' with the accent on the initial syllable", repeating it to challenge intruders.

In March 2023 influenza A virus subtype H5N1 was detected in black-necked swan populations in Carlos Anwandter Nature Sanctuary, Chile and Estación Tapia, Uruguay.

[12][13] As on May 30, 2023, three more black-necked swans were found dead due to influenza H5N1 in Lagoa da Mangueira, Taim Ecological Station, Brasil.