In 1915 the Sienese aristocrat Niccolò Bonsignori left his palace to the council for the establishment of a museum. This is now one of the most important pinacotecas or picture galleries in Italy for the richness, variety and quality of its exhibits. The collection of Sienese masterpieces from the fourteenth and fifteenth centuries is based on the conservation work of Abbot Giuseppe Ciaccheri, who began it in the eighteenth century. In 1932 the Pinacoteca was inaugurated in the rooms of the ancient Palazzo and the adjacent Palazzo Brigidi.

The entrance is simple and elegant and leads to a Renaissance courtyard with stairs to each floor. Next to these stands a Roman sarcophagus. In the courtyard are another sarcophagus and a shrine with a fifteenth century fresco, crowned with an antique washbasin.

The order of the rooms is a chronological exploration of one of the loftiest schools of Medieval and Renaissance painting: the Sienese school.

The visit starts on the second floor where the rooms contain early Sienese works from the twelfth and thirteenth centuries.