Björkö is an island in Lake Mälaren in eastern-central Sweden. It is mostly famous for housing the excavation site Birka populated c. 750-975 CE, together with Hovgården on Adelsö declared a World Heritage Site in 1993. Located just south of Adelsö and west of Ekerö and Munsö islands, it is one of the islands in Ekerö Municipality.

Mälaren, historically referred to as Lake Malar in English, is the third-largest freshwater lake in Sweden. Its area is 1,140 km2 and its greatest depth is 64 m. Mälaren spans 120 kilometers from east to west. The lake drains, from south-west to north-east, into the Baltic Sea through its natural outlets Norrström and Söderström and through the artificial Södertälje Canal and Hammarbyleden waterway. The easternmost bay of Mälaren, in central Stockholm, is called Riddarfjärden. The lake is located in Svealand and bounded by the provinces of Uppland, Södermanland and Västmanland. The two largest islands in Mälaren are Selaön (91 km2) and Svartsjölandet (79 km2). Mälaren is low-lying and mostly relatively shallow.