Adelsö is an island in the middle of Lake Mälaren in Sweden, near Björkfjärden. The island is part of Ekerö Municipality and Stockholm County. It is about 45 kilometres by ferry and road west of downtown Stockholm. The administrative center of the important Viking settlement Birka was situated at Hovgården on Adelsö. The road network is connected via a ferry link across a narrow strait to the main island of Ekerö.