St Mary's Church is the only fully intact medieval church in Sigtuna, a brick Dominican structure from the 13th century that has been in continuous use as a parish church since its construction. The interior retains original medieval features — carved stone, fragments of medieval fresco, and a font that dates from the church's founding period.
The surrounding churchyard is worth exploring: several rune stones have been relocated here for preservation, and the yard is one of the quieter spots in town when the main street is busy. The church is generally open to visitors during daylight hours. It sits just off Stora Gatan and is the natural anchor of any walk through town.