The Museum of Religious Art in Massa Marittima was made possible by close cooperation between the local authorities and the Superintendency for the Artistic, Historical and Environmental Heritage of the Provinces of Siena and Grosseto. Important works dating from the most significant period in the town's history can now be admired in a single museum housed in the restored former 13th century monastery of San Piero Pietro all'Orto. Thanks to the Diocese of Massa Marittima - Piombino, the Museum can count on temporary loans of important works of art belonging to churches in the town, including the Cathedral. This will enable periodic setting up of new artistic itineraries and further research in religious iconography and liturgical tradition.

The collection is housed in five large rooms. The first contains ten grey alabaster bas-reliefs from a very ancient town church, perhaps the excavated chapel in the Old Town. The second room houses gothic sculpture. A reconstruction using original materials of the upper loggetta from the Cathedral façade contains three original sculptures by Giovanni Pisano (a griffon, the ancient Greek hero Telamon and a horse) replaced by copies on the façade itself. A wooden crucifix from tha Cathedral by Pisano can also be seen. Two long ledges hold Apostles, Prophets and Saints by Goro di Gregorio from the Cathedral crypt.
The third room is devoted to the Lorenzetti brothers, with the splendid panel painting of the Maestà or Madonna in Glory by Ambrogio Lorenzetti, dating from the first half of the 14th century, probably done for the adjacent church of San Pietro all'Orto. There is also a window from the Cathedral vestry based on a preparatory drawing by Pietro Lorenzetti.
A large room on the upper floor displays several paintings by Sano di Pietro and Stefano di Giovanni known as Sassetta, as well as two marble tomb slabs, one of which is by Urbano da Cortona, a pupil of Donatello, and other 15th century works.
The last room houses gold objects and church vestments.