An open air museum is a distinct type of museum exhibiting its collections out-of-doors. Like other museums, an open air museum is a non-profit making, permanent institution in the service of society and of its development, open to the public, which acquires, conserves, researches, communicates and exhibits, for purposes of study, education and enjoyment, the tangible and intangible evidence of people and their environment. Most open air museums specialize in the collection and re-erection of old buildings at large out-door sites, usually in settings of re-created landscapes of the past. Most of them may therefore justly be described as building museums. Most open air museums are located in regions where wooden architecture prevails, as wooden structures may be translocated without substantial loss of authenticity.

The earliest ones appeared in Scandinavia in the late 19th century. The reason may be the ancient tradition of moving and re-erecting wooden buildings, based on the local log building technique. The idea was a predictable further development of the by then well-established in-door type of museum. In order to collect and display whole buildings, it would have to be done outdoors. Precursors of open air museums were the "exotic" pavilions, "antique" temples, "ancient ruins" and "peasant cottages" to be found in 18th century landscape parks. Later precursors were the real or constructed peasant cottages shown at the international world exhibitons of the late 19th century.

The world's first open air museum was King Oscar II's collection near Oslo in Norway, opened in 1881. The original plans comprised 8 or 10 buildings intended to show the evolution of traditionla Norwegian building types since the middle ages. But only 5 were realized before the king lost interest because of the expenses involved. The royal open air museum was later incorporated into the Norsk Folkemuseum, established on an adjacent property in the 1890es. Influenced by a visit to the Norwegian open air museum, Arthur Hazelius in 1891 founded the famous Skansen in Stockholm, which became the model for subsequent open air museums in Northern and Eastern Europe, and eventually in other parts of the world.

Around 1900 national and regional open air museums were established in all Scandinavian countries, notably in Norway and Sweden.

A more recent but related idea is realized in the ecomuseums, which originated in France.