The Church of Saint Mary is Sønderborg's main church, a Gothic brick building rebuilt several times since the 15th century after fires and bombardments. The interior is a slightly austere Lutheran whitewash, with a Baroque altarpiece and good 18th-century epitaphs along the walls.
The churchyard around it is one of the quietest spots in the centre of town, with old chestnut trees and benches looking back at the brickwork. The church tower is occasionally open and gives a clean view down to the harbour.