The Piccolomini Library is in the left transept of the Siena Cathedral. However, it deserves to be represented as a separate sight, considering its exceptional artistic value.

Piccolomini Library is known for the wall and ceiling frescoes painted by Pinturicchio, famous Italian painter of the early Renaissance. The library was built in 1485 by cardinal Francesco Piccolomini who eventually became Pope Pius III for only 27 days.

After the death of the notorious Pope Alexander IV there began the fierce fight for the Holy See. Three contenders from France, Spain and Italy was supported by the armed forces of the corresponding states. Particularly, cardinal Francesco Piccolomini was supported by Cesare Borgia. The latter insisted on the election of Piccolomini who later was crowned as Pope Pius III. Pope Pius III died on October 18, 1503. It was strongly rumoured that he was poisoned but that speculation was supported by no evidence.

In 1507 Pinturicchio and his assistants painted 10 giant frescoes, which depicted 10 remarkable events from the secular and religious career of Enea Silvio Piccolomini (Pope Pius II), on the walls of the library. For example, one of the frescoes on the left wall depicts the canonization of a local girl Catherine in 1461.

The famous statue «Three Graces», a Roman copy of a Greek original, is in the middle of the library. The altar of Piccolomini, to the left from the entrance to the library, was created by sculptor Andrea Bregno in 1483. The young Michelangelo created four sculptures for the altar during 1501-1504.