It is housed in a huge vaulted space that is 30 by 9 meters and almost 11 meters high. It is situated beneath the Patio de los Evangelistas in the eastern portion of the cloister and is lit by the patio through five ground-level windows. The center piece is a lovely mirror with a baroque silver frame with rock crystal accents that was a gift from Queen Mariana of Austria (the mother of Charles II), and there are six more, smaller mirrors that are conveniently spaced apart and have silver plate frames that are expertly crafted. The floor is built of white and grey marble, and the vault is decorated with grotesques. Niccol Granello and Fabrizio Castello created the huge coffered ceilings with a variety of embellishments between raised bands.

It has an excellent collection of artwork, including pieces by Luca Giordano (Christ crucified), Titian (Christ in the Garden Prayer), José de Ribera (Saint Peter in Prison), Michel Coxcie (The Virgin, the Infant Jesus, and Saint Anne offering him a piece of fruit), Herrera Barnuevo, and Luca Giordano's false prophet Balaan (Saint John the Baptist). The Adoration of the Sacred Form, a masterpiece by Madrid-born Claudio Coello, stands out among them all. It depicts the religious ceremony held on October 19, 1680, for the solemn transfer of the Holy Form from another area of the Monastery to its new Chapel in the sacristy. The composition is admirably composed both in terms of perspective and the mastery of the figures portrayed: Charles II, who is knelt, Father Fray Marcos de Herrera,

Coello's artwork, which serves as a veil or transparency for the Blessed Sacrament and is only removed once a year, covers the altar entirely (the last Sunday in September). The painting is then removed from the floor, revealing the magnificent crucifix, created by Pietro Tacca, with the figure of Christ exquisitely modeled and cast in fire-gilded bronze, as well as a sizable temple, also fire-gilded, in the Gothic-Germanic style, standing 1.60 meters tall and designed by Vicente López, which was started in 1829 by Ignacio Millán and completed in 1834. It is embellished with forty statuettes and ten busts and holds numerous artifacts. Sadly, one of the gems that vanished in 1936 was the extremely expensive monstrance of Isabella II, a gift from the Queen in 1852 fashioned in Madrid by Carlos Pizzala with magnificent goldsmith work and set with valuable stones. Behind the marble and bronze-accented altarpiece in the sacristy is the dressing room, which was created by Francisco Rizi, José del Olmo, and Francisco Filippi.