Located in the south of the old town core, St. Mary Formosa is one of the most significant Early Christian monuments of the Byzantine art and architecture in Istria and Croatia. The basilica dates from the 6th c. It was commissioned by Archbishop Maximianus of Ravenna from the vicinity of Rovinj, who also had San Vitale and San Appolinare in Classe erected, both in Ravenna.
The magnificent three-nave basilica was divided into three naves by columns, which interior rhythm was repeated on its exterior perimetral walls, the window division and blind arch lesenes. The sanctuary was completed by three polygonal arches and two side chapels next to it. Only the south chapel has been fully preserved until today, while the major part of the northern one was built into the neighbouring residential buildings. The basilica's northern wall is today visible only as a fence surrounding the neighbouring garden. The chapel is designed as a Greek cross, one of which arms ends in a semi-circular axis, while its central part, the point where the two arms cross, is higher than the others. The sanctuary is covered by the quadro-pitched roof, while the remaining part of the structure has dual-pitched roofs. The exterior is simple, decorated by shallow lesenes, blind arches and semi-circular windows.
It got ruined, especially during the 1242 fire at the time of the Venetian conquest of Pula. A large portion of its inventory was shipped to Venice, where it was used in building the St. Mark's Library or Sale delle quattro porte of the Doge's Palace. In the late 16th c., the basilica was in ruins.