The majestic sacristy, the place where priests prepare for the celebration of the Eucharist and where the liturgical decorations and the sacred vessels are preserved, has been almost entirely decorated by Andrea Sacchi in 1600. The canvas which represents the crucifixion is also of him, is placed at the bottom and centre of the vestibule. Apart from Sacchi, the painter Giuseppe Puglia also helped with decorating the imposing canvas of St. Dominic in glory, positioned in the centre of the vault.

The fresco of the Roman painter G. B. Speranza is placed on the front door, dating from 1640, and represents two conclaves which took place here, when the Supreme Pontiffs Eugene IV, in 1431, and Nicolò V, in 1447 were elected. It is certain that these two conclaves took place there, but there were certainly many other popes elected in the Basilica of Minerva.

We note that groups of three bees appear in several places: they are the emblem of Cardinal Antonio Barberini. The Barberini family has been a great benefactor of the Dominican Order, and they funded the decor of the sacristy.

The furniture dates from the seventeenth century and it is entirely in walnut of very great value.

The room where Saint Catherine died 29th of April 1380 is  behind the sacristy, which can only be visited by appointment. We can catch a glimpse of it through the railings at the bottom of the sacristy. Originally this room was accessed from the « Santa Chiara » square, near the front of the Basilica, but cardinal Barberini dismantled it in order to build a larger chapel that he rebuilt in the Basilica by carrying the walls that witnessed the transit of the saint from Siena. The chapel, in addition to its spiritual value, is enriched with magnificent frescoes of Antoniazzo Romano, the author of the Annunciation altarpiece (see chapel of the Annunciation n. 3) And of his students.

In the corridor that leads to the sacristy there are two doors, one of which is the monumental entrance to the Casanatense library (whereas the entry for the tour is at « via di Sant'Ignazio »), and the other that of the "pope’s room" where a magnificent and unfinished statue of the Virgin Mary with the child is preserved, attributed to Bernini. The Casanatense library, founded by the theologian Cardinal Casanate has a rich collection of more than 300,000 volumes, and a magnificent monumental lounge realised by Carlo Fontana.