Bishop Rolando, who supported Innocent II during the schism of Anacletus, obtained authorisation to transfer the seat of the See of Rosellana to Grosseto from Innocent II on 9 April 1138. During the 12 C, the parish church of Santa Maria Assunta was the cathedral of Grosseto. It was located more or less in the position the apse of the current cathedral which was finished in 1294, as shown by two dated stone in the façade and inside of the church, one dated 1294 and the other 1295. The present Cathedral of Grosseto is dedicated to San Lorenzo and was built under the supervision of the Siennese Sozzo da Rustichino. The belfry tower was added in 1402 during a period of Siennese domination. (It was restored and modified in 1911.)
The façade has lost its original appearance, having been completely rebuilt between 1816 and 1855, but some evidence of the original cathedral has been preserved, notably the symbols of the Evangelists. Between 1859 and 1865, another major restoration gave to the inside of the cathedral a "neo-gothic" appearance that it did not have previously.
The major art works are a Baptism Font and the altar of the Madonna of the Graces, both the work of Antonio di Paolo Ghini from between 1470 and 1474, the marvelous Madonna of the Graces by Matteo di Giovanni, also from 1470, and the right side of the cathedral which is in the Siennese style.