Adelsö is an island in the middle of Lake Mälaren in Sweden, near southern and northern Björkfjärden. The administrative center of the important Viking settlement Birka (on the neighbouring island Björkö) was situated at Hovgården on Adelsö.
The history of Adelsö began with the Stone Age. Adelsö at that time consisted of small islands which emerged from the sea at the end of the Ice Age. Mälaren, a freshwater lake, did not yet exist, so the skerries that were to become Adelsö lay beneath the Baltic Sea.
Fishing, bird- and seal-hunting created the foundation for the life of the people living there. Graves from the early and late Stone Age exist, but most of the gravesites date from the Iron Age, mostly the Viking Age. There are also two ancient hillforts (fornborgar) on Adelsö; one of them, situated on Skansberget near Stenby, is unusually well-preserved.
Adelsö, earlier called Alsnö or Alsnu reflects the importance of the area during the Viking Age. The King's House (Kungsgården) was next to Hovgården and the monarch ruled over the nearby Viking city of Birka. This is also the site of Kungshögarna at Hovgården. Kungs means king or royal and högarna, from the Old Norse word haugr, means mound or barrow.
During the latter part of the 12th century, a Christian church was built next to Hovgården. Birger Jarl's sons built Alsnö hus, a splendid castle where King Magnus I in 1279 assembled the Meeting of Alsnö. At that meeting, the Ordinance of Alsnö was established, introducing the privileges of the Swedish nobility.
During the Middle Ages, Alsnö hus was used as a summer palace for kings and governors but later fell into ruin. What remains of the castle and several graves near Hovgården was excavated during archeological digs conducted between 1916 and 1926.