The interior has a basilica plan with an apse and two aisles, divided by cylindrical columns and cruciform pilasters which end in complex composite capitals. Next to the entrance door, under the 14th century triptych of the Madonna Enthroned with Child and Saints, is a Roman sarcophagus from the 4th century. On the right of the counter-façade is a series of panels in soft stone, dating from the High Middle Ages or, according to some sources, to the 12th-13th century. Over the panels are frescoes of the Crucifixion, of the 14th century, and the legend of Saint Julian, of the 15th century.
The central rose window has rare stained glass of the Sienese School. On the left is the monumental baptismal font, with a rectangular bath surmounted by a small temple. The baptistery dates from 1267.
The south aisle houses paintings/sculptures of the Madonna in Glory by Antonio Nasini (late 17th century) and the Nativity of the Virgin by Rutilio Manetti (16th century). In the chapel on the right of the presbytery is a painted cross by Segna di Bonaventura (14th century). The wooden pulpit is of the 17th century. Also by Rutilio Manetti are the paintings/sculptures of the Immaculate Conception and the Eternal Father in the chapel to the right of the major one.
The high altar, in marble (1626), has a polychrome wooden crucifix by Giovanni Pisano (early 14th century). At the foot of the altar are two wooden angels from the 15th century, by Domenico di Niccolò dei Cori. Behind it is the Ark of Saint Cerbonius, which was created in 1342, with reliefs depicting stories of the saint.