Tre Kronor or Three Crowns Castle was a castle located in Stockholm, Sweden, on the site where Stockholm Palace is today. It is believed to have been a citadel that Birger Jarl built into a royal castle in the middle of the 13th century. The name "Tre Kronor" is believed to have been given to the castle during the reign of King Magnus IV in the middle of the 14th century.

Stockholm is the capital and most populous city of Sweden as well as the largest urban area in the Nordic countries. Approximately 1 million people live in the municipality, with 1.6 million in the urban area, and 2.4 million in the metropolitan area. The city stretches across fourteen islands where Lake Mälaren flows into the Baltic Sea. Outside the city to the east, and along the coast, is the island chain of the Stockholm archipelago. The area has been settled since the Stone Age, in the 6th millennium BC, and was founded as a city in 1252 by Swedish statesman Birger Jarl. The city serves as the county seat of Stockholm County.