The sacristy is reached by way of the chapel of St. Peter. It is independent of the rest of the original building and was built in 1486 by director of works Giorgio Spavento. From 1524 the most celebrated artists of the age worked on its embellishment.

The vault and wall lunette mosaics make up a homogeneous decorative group dating to the first half of the 16th century (1524-1530), notable for splendour and technical skill but by that time wholly dependent on painting. They are the work of Alberto Zio,Marco Luciano Rizzo and Francesco Zuccato, the foremost mosaic masters of the day.
The mosaic decoration of the upper part of the walls fits in and gives unity to the renaissance purity of the architecture. Some attribute the design to Titian.

The Christological plan has its centre in the vaulted ceiling with its great cross and Christ with the Four Evangelists. All around are the Prophets who had announced the coming of Christ. In the lunettes of the two main walls the Twelve Apostles with St. Mark and St. Paul who had borne witness to him in the world, a work carried out to a cartoon by Titian. Above the door, the Eternal Father in Glory. On the west side the Virgin and Child are flanked by St. George and St. Theodore.
Scholars acknowledge that the vault mosaics are of the highest quality.