Originally called Santa Maria del Canetto because of its marshy location, it was then called Santa Maria della Mercato thanks to the market held in the square facing the church. Following the transition from the rule of the consuls to that of a single Podesta, in the Middle Ages, the square was used for the investiture of the latter before the people.

The church is a jewel of Romanesque architecture, with a rectangular plan in the shape of a Latin cross divided into three naves. Steps lead to a raised altar. The portal is embellished with ornaments, plus an interweaving pattern of small arches and pillars.

In 1980, restoration work brought to light the remains of the earlier basilica as well as those of Greek walls dating back to the fourth century BC.