Begun in 1882 according to the original design by Francisco del Villar, when Gaudí took control of the construction on November 3, 1883, he introduced changes to the pillars, adding capitals with naturalistic motifs. In addition, he raised the vault and surrounded the crypt with a moat to ensure the entry of light and direct ventilation. On the other hand, he relocated the main altar from its intended position on the main staircase to the center of the transept, thus leaving the space clear and placing two spiral staircases on the sides. The first plans that Gaudí drew up for the Sagrada Familia corresponded to the chapel of St. Joseph, built between 1884 and 1885, the year in which the first mass took place. The completion of the work on the crypt lasted until 1891.

The crypt, located 10 meters deep from street level, has a semicircular shape and measures 40 meters long by 30 meters wide. The ambulatory houses seven chapels dedicated to the Holy Family of Jesus: St. Joseph, the Sacred Heart, the Immaculate Conception, St. Joachim, St. Anne, the chapel of St. John the Baptist and St. John the Evangelist, and the chapel of St. Elizabeth and St. Zachary. These chapels are arranged in a circular design. In front of them are five other chapels in a straight line: the central chapel, dedicated to the Holy Family and which houses the altar; flanked by the chapels of Our Lady of Carmen, where Gaudí is buried; of Jesus Christ; of Our Lady of Montserrat; and of the Holy Christ, where Josep Maria Bocabella was initially buried until his tomb was desecrated in 1936. Under the spiral staircases on the sides are the corresponding sacristies.

The vaults of the crypt adopt a Gothic style, each one of them -in a total of 22- has a central keystone decorated with anagrams or figures of angels and other elements. The central keystone of the vault stands out, which presents a polychrome relief representing the Annunciation, a creation of Joan Flotats. This central vault is the highest and protrudes two meters above the floor, at the height of the presbytery, where large windows allow observing the crypt from above and provide illumination to the space below. The vaults are supported by lunettes resting on arches supported by ten pillars arranged in bundles of small columns.