Mariakyrkan (St. Mary's Church) was built as a convent church the middle of the 13th century by the Dominican Order. It is the oldest building still in use in Sigtuna and the walls, pillars and roof of the church look much the same as they did then.

Mariakyrkan was built in a transitional style between Romantic and Gothic styles and got a completely different look than the stone churches. It’s size, the color of the bricks and the light in the church room must have made a divine impression on the visitors. It is the first known brick church in the Lake Mälaren area. The bricks were made in ovens to the north of the church.

During the Reformation around 1530 the convent was closed, and the Dominicans were driven away from Sigtuna. Mariakyrkan became a parish church. It is the only medieval church in Sigtuna which survived the Reformation. The two baptismal fonts are the oldest items in the church and there are also two well-kept medieval altarpieces. The four crosses were painted on the walls in the 13th century where the bishop had splashed holy oil during the inauguration of the church.