The church of San Pietro, to date, is the oldest church remained in Grosseto of all those existing in the historic center. It had a great importance in the low-income history of the city and was mentioned in the bull of Pope Clement III of 1188 . The data of the excavation carried out between 2004 and 2005 has allowed us to read a longer and more complex history that dates back to the Early Middle Ages and closely links the events of the church to those of the city.
The gabled façade is original up to about 4.50 m high, with the portal framed by pilasters and the diamond-shaped recesses, inspired by Pisa, which contained ceramics now lost; higher up the mullioned window and the arches are attributed to the heavy restoration of 1911.
The present church should have been built immediately after 1138, the date of the move of the Episcopal See from Roselle to Grosseto.
 The bell tower was built in its present form in 1625 using the foundations of a previous bell tower or an element in phase with the church of the late ninth century.
Inside the restorations have eliminated the coarse interventions of 1952, and in particular the refoderatura of the walls.