The Greylag Goose, Anser anser, is a bird with a wide range in the Old World. It is the type species of the genus Anser.

It was in pre-Linnean times known as the Wild Goose ("Anser ferus"). This species is the ancestor of domesticated geese in Europe and North America. Flocks of feral birds derived from domesticated birds are widespread.

The Greylag Goose is one of the species to which the Agreement on the Conservation of African-Eurasian Migratory Waterbirds (AEWA) applies.

The Greylag is a large goose, 74–84 cm (29–33 in) long with a 149–168 cm (59–66 in) wingspan and a body weight of 2.3–5.5 kg (5–12 lbs). It has a large head and almost triangular bill. The legs are pink, and the bird is easily identified in flight by the pale leading edge to the wing. It has a loud cackling call, kiYAAA-ga-ga, like the domestic goose.

The western European nominate subspecies, A. a. anser, has an orange-pink bill and is slightly smaller and darker than the pink-billed Asian race, A. a. rubrirostris. Eastern European birds are often intermediate in appearance.

This species is found throughout the Old World, apparently breeding where suitable localities are to be found in many European countries, although it no longer breeds in southwestern Europe. Eastwards it extends across Asia to China.

The geese are migratory, moving south or west in winter, but Scottish breeders, some other populations in northwestern Europe, and feral flocks are largely resident. This species is one of the last to migrate, and it is thought that "greaylag" signifies in English "late", "last", or "slow", as in laggard, a loiterer, or old terms such as lagman, the last man, lagteeth, the posterior molar or "wisdom" teeth (as the last to appear), and lagclock, a clock that is behind time. Thus the Greylag Goose is the grey goose, which in England when the name was given, was not strongly migratory but lagged behind the other wild goose species when they left for their northern breeding quarters.

In Great Britain their numbers have declined as a breeding bird, retreating north to breed wild only in the Outer Hebrides and the northern mainland of Scotland. However during the 20th century, feral populations have been established elsewhere, and they have now re-colonised much of England. The breeding habitat is a variety of wetlands including marshes, lakes, and damp heather moors.

Within science, the greylag goose is most notable as being the bird with which the ethologist Konrad Lorenz first did his major studying into the behavioural phenomenon of imprinting.

In North America, small populations of Greylag Geese descended from domesticated geese have become established, mostly in city parks and near humans. These geese usually exist as part of larger flocks of Canada Geese. The Greylag Goose can hybridize with the native Canada Goose, producing birds which can be puzzling to birders attempting to identify them.