Built in 1038, it rises on the coastline on the remains of a previus episcopal church of the Graces (VIII century), of which we can admire some frescos in the Vescovado Square. The cathedral is Romanesque lagoon style and is divided into three naves by two lines of alternate columns and pillars which both terminate in a circular apse.

On the sides of the central doorway there are two tall bass-reliefs representing St.Agatonico, whose name appears clearly in Greek, and St. Guglielmo of Tolosa treading on a snake. The church is dedicated to St.Stephen, first of many Cristian martyrs, whose skull is conserved in the church.

In the middle of the central apse there is an altar piece with six panels in gold plated silver. Legend relates that it was a gift from Queen Caterina Cornaro who landed in Caorle miraculously escaping a terrible storm whilst returning to Cyprus. Two panels, with Greek inscriptions,portray the Archangel Gabriel and the Annunciation to the Virgin Mary,evidence of XII century Byzantine origin.On both sides of the altar table there are six panels illustrating saints which were part of the division separating the dignitaries from the faithful.

The interior is divided in three naves,where the main one is double the size of the lateral ones and is divided in 18 columns and pillars,that sustain the ten arches towards the presbytery, the apse and the main altar. The west side appears unitarian without any windows whilst on the east side there are seven oblong windows. The number seven, which also appears on the outside with the seven arches of the central apse,is a mystic number of biblical derivation.