Swans are birds of the genus Cygnus within the family Anatidae. The swans' closest relatives include geese and ducks. Swans are grouped with the closely related geese in the subfamily Anserinae, forming the tribe Cygnini. Sometimes, they are considered a distinct subfamily, Cygninae. They are the largest waterfowl and are often among the largest flighted birds in their range.

There are six living and many extinct species of swan; in addition, there is a species known as the coscoroba swan, which is no longer considered one of the true swans. Swans usually mate for life, although separation sometimes occurs, particularly following nesting failure, and if a mate dies, the remaining swan will take up with another. The number of eggs in each clutch ranges from three to eight.

The genus Cygnus was introduced in 1764 by the French naturalist François Alexandre Pierre de Garsault. The English word swan, akin to the German Schwan, Dutch zwaan, and Swedish svan, is derived from the Indo-European root *swen(H) ('to sound, to sing').

Young swans are known as cygnets, from Old French cigne or cisne (diminutive suffix et 'little'), from the Latin word cygnus, a variant form of cycnus 'swan', itself from the Greek κύκνος kýknos, a word of the same meaning. An adult male is a cob, from Middle English cobbe (leader of a group); an adult female is a pen. A group of swans is called a bevy^{[citation needed]} or a wedge.

Swans are the largest extant members of the waterfowl family Anatidae and are among the largest flying birds. The largest living species, including the mute swan, trumpeter swan, and whooper swan, can reach a length of over 1.5 m (59 in) and weigh over 15 kg (33 lb). Their wingspans can be over 3.1 m (10 ft). Compared to the closely related geese, they are much larger and have proportionally larger feet and necks. Adults also have a patch of unfeathered skin between the eyes and bill. The sexes are alike in plumage, but males are generally bigger and heavier than females. The biggest species of swan ever was the extinct Cygnus falconeri, a flightless giant swan known from fossils found on the Mediterranean islands of Malta and Sicily; its disappearance is thought to have resulted from extreme climate fluctuations or the arrival of superior predators and competitors.

The Northern Hemisphere species of swan have pure white plumage, while the Southern Hemisphere species are mixed black and white. The Australian black swan (Cygnus atratus) is completely black except for the white flight feathers on its wings; the chicks of black swans are light grey. The South American black-necked swan has a white body with a black neck.

The legs of most swans are typically a dark blackish-grey colour, except for the South American black-necked swan, which has pink legs. Bill colour varies: the four subarctic species have black bills with varying amounts of yellow, and all the others are patterned red and black. Although birds do not have teeth, swans, like other Anatidae, have beaks with serrated edges that look like small jagged "teeth" as part of their beaks, which are used for catching and eating aquatic plants and algae, as well as molluscs, small fish, frogs, and worms. In the mute swan and black-necked swan, both sexes have a fleshy lump at the base of their bills on the upper mandible, known as the knob, which is larger in males and is condition-dependent, changing seasonally.

Swans are generally found in temperate environments, rarely occurring in the tropics. Four (or five) species occur in the Northern Hemisphere, one species is found in Australia, one extinct species was found in New Zealand and the Chatham Islands, and one species is distributed in southern South America. They are absent from tropical Asia, Central America, northern South America and the entirety of Africa. One species, the mute swan, has been introduced to North America, Australia and New Zealand.