St Mark's Sq (Trg Svetog Marka) is dominated by the magnificent St Mark's Cathedral. This 15th-century cathedral was built from Korčula limestone in a Gothic-Renaissance style by Italian and local artisans. Over the solemn portal, the triangular gable cornice is decorated with a two-tailed mermaid, an elephant, and other sculptures. The bell tower that rises from the cathedral over the town is topped by a balustrade and ornate cupola, beautifully carved by the Korčulan Marko Andrijić.

The interior of the cathedral features modern sculptures in the baptistry, including a pietà by Ivan Meštrović. The ciborium was also carved by Andrijić and behind it is the altarpiece painting Three Saints by Tintoretto. Another painting attributed to Tintoretto or his workshop, The Annunciation, is on the baroque altar of St Antony. Other noteworthy works include a bronze statue of St Blaise by Meštrović near the altar on the northern aisle, and a painting by the Venetian artist Jacopo Bassano in the apse of the southern aisle.