The Saint Nicholas Cathedral, the seat of the Serbian Orthodox eparchy of Karlovac and Slavonia, is a handsome 19th-century church on Strossmayerov trg. The dim interior is alive with icons, frescoes and a tall iconostasis that contrasts sharply with the plain baroque of the neighbouring Catholic churches.
Karlovac's Serbian community has been an important part of the city since the 17th century. The cathedral is open for services and to visitors at quieter hours.