It was in this division that the canons regular kept their vestments and other liturgical implements, and also prepared themselves for the celebration of religious ceremonies.

As usual, the sacristy is located next to the church, however here is placed in a very original way, in the Portuguese context: incorporated in the plan of the monastery between two cloisters. The sacristy and its portal were built in the 1st half of the 18th century, and are credited to the architect Luís Nunes Tinoco. Inside, the walls are richly decorated with inlaid polychrome marbles, which dignify the space.

In the interior, it is also possible to observe: two large cabinets made of jacaranda wood (from Brazil), a bust in relief of King D. João V, busts of several bishops and an altar decorated with a magnificent painting in oil on canvas by André Gonçalves, entitled Our Lady and the Saints (c.1760).

The ceiling was rebuilt after the earthquake of 1755, and is it now painted in oil on canvas glued on wood. At the centre it is possible to see the representation of the Agnus Dei (Lamb of God) and the side medallions display iconography related to the Monastery.

Beneath the pavement were found anthropomorphic tombs that date back to the 12th century, where the knights from the Second Crusade, who assisted King D. Afonso Henriques in the conquest of Lisbon, may have been buried.