Co-Cathedral of the Assumption of Mary, Senj, Romanesque co-cathedral in Senj, Croatia.

The Co-Cathedral of the Assumption of Mary is a three-nave church whose Romanesque form dates to 1250, with a facade decorated by brick arches and niches influenced by northern Adriatic building. The interior was given its neo-Baroque character under Bishop Mirko Ožegović. Inside are the Gothic tomb of Bishop Ivan Cardinalibus from 1392 and a 1492 relief of the Holy Trinity bearing the Perović coat of arms in a chequerboard pattern.

The church was completed in 1169, when a separate Diocese of Senj is first recorded, with Mirej as its first bishop. It was built on the foundations of a 4th- or 5th-century early Christian basilica. After severe damage in 1239 fighting the Mongols, it was rebuilt by 1250 in Romanesque form; in 1248 Pope Innocent IV allowed Mass in Old Church Slavonic and Glagolitic script.

The church is the co-cathedral of the Gospić-Senj diocese, a role it has held since the Senj diocese passed to the Rijeka archdiocese in 1969. Inside, the pews carry inscriptions in Cyrillic, and the altar of the Holy Angels holds the body of St. Formosus. Above the sacristy door a 15th-century inscription records that Bishop Andrija of Modena added the room.

The church is located in central Senj and is easily accessible on foot. Visitors should know that viewing is possible during religious services, but should be respectful of worshippers present.

The church stands on the foundations of an early Christian basilica from the 4th or 5th century, not a pagan temple. Inside, the Altar of the Holy Angels holds the body of St. Formosus, and the pews carry Cyrillic inscriptions. The 1492 Trinity relief bears the Perović family coat of arms in a chequerboard pattern.