The Co-Cathedral of Saint Mary of the Assumption and Saint Catellus (Italian: Concattedral di Maria Santissima Assunta e San Catello) is the Roman Catholic duomo or cathedral of Castellammare di Stabia in the metropolitan city of Naples, in the region of Campania, Italy.

There was a first attempt in 1517 when a committee of citizens assigned the task of projecting the new building to the architect Giovanni Donadio, but for unknown reasons, the plan failed.

In 1581 the bishop Ludovico Maiorano sold the co-cathedral, henceforth fallen into ruin, and earned a fair amount of money to invest into the construction of the new cathedral.

The new co-cathedral stood on the ruins of the former in Quartuccio zone, and the construction works were assigned to Pietro Antonio De Sanctis, Santoro Cortolano, and Paolo Fasano (the cornerstone laying ceremony is told in one of the books kept in the cathedral's historic archive).

Finally, in 1643, the building was partially opened to the public thanks to the private chapels that were granted to some noble families of the city.

The bishop strongly opposed this because he would have had to give about 2 meters of his land, so in 1782 the king intervened and a new bell tower was built with an expense of 7,323 ducats.

The plan took the shape of a Latin cross, and the transept and presbytery were built together with the chapel for San Catello.

During the excavations of the patron saint's chapel, workers discovered the rest of a Paleochristian necropolis, segments of streets, and doors belonging to ancient Roman homes.

The Chapel of San Catello, dedicated to the patron of the town, was begun in 1875 and completed in 1893 when Bishop Vincenzo Sarnelli reconsecrated the cathedral.

[1] The co-cathedral's facade, decorated with marbles, is opened by three arches leaning on four pillars creating a small entrance hall that you can reach from a large staircase made of a rock called “piperno of Soccavo”.

The entrance to the church is enabled by three bronze doors created by the Florentine sculptor Antonio Berti, which were given as a present in 1985 by the Bank of Stabia.

The pavement is made of black and grey marbles, squared and octagonal, and in the center of the nave there is the ancient stabian priests' grave.

Ten lunettes representing the virtues of the patron saint's life surround the paintings: Reliability, Constancy, Zeal, Long-suffering, Forgiveness, Charity, Prayer, Faith, Love, and Hope.

On the main altar is displayed a reproduction in stucco and cork of the Grotto of Apparitions with the statues of Virgin Mary and Saint Bernadette in prayer.

The first has an octagonal shape and on its main altar is the marble statue of the Saint Archangel (the one previously positioned in the church on Mount Faito).

The statue appears to be of medieval origins, but according to the traditional story, Pope Gregorio Magno gave it to Saint Catello in the 6th century.

The main altar is made of bronze and colored marbles, and below there is the sarcophagus of the Good Shepherd of Early Christian origin (also found during the excavation).

On the wall at the cathedral's end, there is a funerary monument with a sculpture of Tommaso Angelo d'Arco's face and three marble graves.