The greylag goose or graylag goose (Anser anser) is a species of large goose in the waterfowl family Anatidae and the type species of the genus Anser. It has mottled and barred grey and white plumage and an orange beak and pink legs. A large bird, it measures between 74 and 91 centimetres (29 and 36 in) in length, with an average weight of 3.3 kilograms (7 lb 4 oz). Its distribution is widespread, with birds from the north of its range in Europe and Asia migrating southwards to spend the winter in warmer places. It is the ancestor of most breeds of domestic goose, having been domesticated at least as early as 1360 BC. The genus name is from anser, the Latin for "goose".

Greylag geese travel to their northerly breeding grounds in spring, nesting on moorlands, in marshes, around lakes and on coastal islands. They normally mate for life and nest on the ground among vegetation. A clutch of three to five eggs is laid; the female incubates the eggs and both parents defend and rear the young. The birds stay together as a family group, migrating southwards in autumn as part of a flock, and separating the following year. During the winter they occupy semi-aquatic habitats, estuaries, marshes and flooded fields, feeding on grass and often consuming agricultural crops. Some populations, such as those in southern England and in urban areas across the species' range, are primarily resident and occupy the same area year-round.