Sankt Nikolai-Kirche stands in the harbour district, where it has served as the church of Flensburg's merchant community since the medieval period. The current building dates largely from the fifteenth and sixteenth centuries, though there has been a church on this site since at least the thirteenth. The tower is a landmark visible from the water.
The interior is worth entering for the carved altar and the historical atmosphere. The church is typically open during the day and services are held regularly. The surrounding churchyard contains gravestones of prominent Flensburg merchant families, which read as a compact history of the city's commercial elite.