Christianity first spread in cities, establishing its earliest communities there. By ad 425 (following Emperor Constantine I’s Edict of Milan in ad 313, which ended religious persecution in the Roman Empire), a law mandated the establishment of bishoprics in cities. As Krk was already an important city following the advent of Christianity, it became a bishopric early on, like other Mediterranean centres. This allowed for the construction of churches. The Cathedral of Krk, dedicated to the Blessed Virgin Mary, stands as the most significant architectural and artistic monument on the island of Krk. Built in the 5th century as a three-aisled early Christian basilica, it was oriented traditionally east-west and constructed on the remains of Roman thermal baths. The surviving structure includes a three-aisled Romanesque design, with the central nave ending in a polygonal apse and the side aisles in flat apses.

The first known reference to the cathedral appears in a donation by Bishop Ivan in 1186, outlining obligations to the Benedictines. The cathedral, with its long and rich history, has undergone numerous additions over the centuries, with the Frankopan Chapel, an architecturally unique late Gothic addition to the northern wall, drawing particular attention. Inside, above the polygonal layout, a star-shaped vault adorned with shields bearing the coats of arms of the Princes of Krk has been constructed. Given the use of their newer coat of arms, the chapel is believed to have been built after 1430 and is interpreted as a princely investment.

Today, elements of the original basilica, such as the barrel-shaped baptismal font (piscina), floor mosaic remnants, and various stone ornamentations, remain preserved. 

The central section of the cathedral’s floor houses the tomb of Ludovico Cicuta (Ljudevit Čikuta), commander of the Krk galley Cristo Ressussitato, which fought on the victorious side of the Holy League in the famed naval Battle of Lepanto in 1571.

Among the cathedral’s rich inventory are baroque paintings by Cristoforo Tasca (altar area) and Nicola Grassi (the Sacred Heart of Jesus sanctuary). Additionally, massive wooden works, including the pulpit, bishop’s throne, a sacristy wall cabinet, and an altar, were crafted by master woodworkers Ivan Sneit, Mihovil, and Franjo Zierer. The monumental organ in the choir loft was built by Venetian organ maker Gaetano Callido in 1815.

Early Christian churches did not originally have bell towers, and the Krk Cathedral was no exception. The existing bell tower was built only in 1515 in the Gothic style, due to limited space, it was placed adjacent to the side of the Church of St Quirinus. After its pointed top was destroyed by a lightning strike, a new distinctive baroque dome was constructed in 1765, following a design by Venetian architect Pietro Checchi. In 1767, an angel with a trumpet, crafted in Venice, was placed atop the dome.