The Cathedral of Saint Domnius (Croatian: Katedrala Svetog Duje), known locally as the Saint Dujam (Sveti Dujam) or colloquially Saint Duje (Sveti Duje), is the Catholic cathedral in Split, Croatia.
The cathedral is the seat of the Archdiocese of Split-Makarska, headed by Archbishop Marin Barišić. The Cathedral of St. Duje is a complex of a church, formed from an Imperial Roman mausoleum, with a bell tower; strictly the church is dedicated to the Virgin Mary, and the bell tower to Saint Duje. Together they form the Cathedral of St. Duje.
The Cathedral of Saint Domnius, consecrated at the turn of the 7th century AD, is regarded as the oldest Catholic cathedral in the world that remains in use in its original structure, without near-complete renovation at a later date. The structure itself, built in AD 305 as the Mausoleum of Diocletian, is the second oldest structure used by any Christian Cathedral