The construction of the chapel by the Cathedral builder Alberto Aringhieri began in 1482 in order to hold the most renowned relic of the cathedral, donated in 1464 by Pope Pius II Piccolomini in a beautiful silver reliquary: the right arm of Saint John the Baptist, who baptised Jesus in the water of the river Jordan. The furnishings and decoration of the room are therefore centred around the figure of Saint John, the last prophet and Forerunner of Christ, as sent by God to prepare the Jews for the coming of the Messiah. The chapel is dominated by a huge bronze statue depicting Saint John the Baptist, the masterpiece of Donatello’s late artistic output, which was completed in Florence within the year 1457; on the walls the biography of Saint John the Baptist was painted by Pinturicchio between 1504 and 1506. The renowned relic had belonged to Tommaso Paleologo, the despot of Morea and brother of the Emperor of the East. The exhibition of the Arm of Saint John occurs around five days before the festival, which takes place on June 24. On this occasion the relic is moved to the high altar from the small room behind the Chapel of Saint John. The relic is preserved in a reliquary which can only be opened by the insertion of 3 keys: the first key is owned by the Comune of Siena, the second by the Sacristry of the Cathedral and the third by the Opera of the Cathedral.
The relic is displayed from the 19th to the 24th June every year on the occasion of the festival of Saint John.