[1] Today Östhammar Municipality is a part of Uppsala County, but the area has historically been a part of Stockholm County.

Östhammar's city status was granted by king Albert in 1368.

During this period a castle called Östhammarshus was built, but already in 1434 the castle was burnt down by its Danish vogt, who fled to Denmark over the sea when faced with the Engelbrekt rebellion.

By the late 15th century the post-glacial rebound had made it hard to reach Östhammar by boat and its city status was transferred to Öregrund.

Östhammar regained its city status in 1538, and moved closer to the sea.