Decorated during the reign of D. João V, circa 1746, the sacristy has a limestone slab floor in various tones, creating a geometric mosaic, and a flat ceiling with an enormous painting depicting "The Assumption of the Virgin" by André Gonçalves (b. 1692 - d. 1762). The walls are lined in polychrome azulejos forming an ashlar, representing on one side the royal coat of arms and on the other the Franciscan coat of arms. The azulejo panels are topped by painted canvases alluding to the life of Joseph of Egypt. Note the large rosewood chest from the XVIII century, with its two-tiered back with 16th century paintings depicting Saint Lucia, Saint Agnes and Saint Catherine.