The learned Archdeacon Sallustio Bandini, who had formed in his home a library open to all scholars, donated his books to the University of Siena in 1759. On that occasion the library found a more appropriate site in the building that had previously been occupied by the Academy of the "Intronati" (the dazed ones).

The Academy was officially constituted in 1525, with the objective of promoting the arts, literature and history. In 1729 it moved to the hall adjacent to the Sapienza, today occupied by the Municipal Library. The Academy, still active today, headquartered in Palazzo Piccolomini, is dedicated to promoting study of the historical, artistic, and literary heritage of Siena.

The Intronati Library became the property of the Commune during the period of Napoleonic rule, subsequent to the suppression of the university. It consists of approximately 500,000 volumes, among them 5,000 manuscripts and 1,000 incunabula. Made up of private collections and monastery sources, it includes numerous illuminated manuscripts, a rich group of drawings and prints, ancient juridical documents as well as interesting manuscripts and drawings on Sienese technology and engineering, and numerous musical codexes.