It is the church of the Assumption oft he Blessed Virgin Mary, a Romanesque structure extremely rich inventory and, historically, an important episcopal center in the area of the northern Adriatic. It was built at the bottom of the hill on which lies the city of Krk. The first known dated mention of this cathedral is located in the grant of bishop Ivan in 1886., but the existence is possible in the early Christian period, another in the second half of the fifth and early sixth century.

Today's cathedral sequence the various renovation and upgrading. Many valuables are decorating the interior space. Among those is a big marble altar, Gothic statue of the Virgin and Child, gold-plated silver frankopan pala depicting Madonna in Glory, Corinthian type stone capitals adorned with unique ancient Christian symbol oft he Eucharist- a bird that feeds on fish, gravestones of Krk bishops, wooden pulpit, paintings of Italian masters. Here is the tomb(2002.) of Bishop Antun Mahnić, the author of the Croatian Catholic Movement, founder of the Old  Academy. In the cathedral during the summer months are traditionally held concert events.

Krk cathedral with Frankopan castle are visually most attractive buildings in the city. It is not only a cathedral  but a unique complex of several religious buildings: the Cathedral of Assumption, St. Quirinius and St. Margaret. During recent excavations, along the northern part of the cathedral was discovered Roman Spa from the 1st century. The white floor mosaic baths preserved in tanks, while other parts are still hidden in the ground. According to some sources, in the 3rd or 4th century the parts of the Spa were already renovated as a Church, and at the end of 5th century the early Christian basilica was built that is the backbone of today's cathedral. As was the custom at the beginning of Christianity, the baptistry was located outside the corpus of the church, and as such was used until 1565. Later the baptistry was connected to the space of the church, and was discovered in 1966 when a pool got covered with  a plate with the inscription: Piscina antiqui baptisterii..