The Basilica of the Immaculate Conception can hold 600 worshippers. 51 metres long and 21 metres wide, it has a single nave. It rises 20 metres above the River Gave that flows past on the right. The basilica's nave is completely encircled by nineteen stained-glass windows. They illustrate scenes concerning the Immaculate Conception, from the beginning of mankind right up to the dogma's proclamation by Pope Pius IX which took place on 8 December 1854. On the upper left: the expulsion of Adam and Eve from the Garden of Eden after committing original sin; then scenes from the Old Testament symbolising the Immaculate Conception. On the upper right: a depiction of events from the New Testament and the history of the church. The windows were designed and constructed by Laurent Gsell under the guidance of Canon Lambert of Paris.
The columns and arcades are made of stone from Angouleme. The altar of white Carrara marble is directly over the grotto of the apparitions. On the right of the presbytery is the white Carrara marble statue of Our Lady of Lourdes. Sculpted by the French sculptor, Emilien Cabuchet, it was placed in the basilica in 1876 during the ceremony of the coronation of Our Lady of Lourdes. The organ, made of Russian oak, is composed of 25 organ-stops.
The exterior is dominated by a rectangular 70 metre gothic spire which stands 100 metres above the River Gave. Above the entrance is a mosaic depicting Pope Pius X. The basilica clock plays the "Ave Maria of Lourdes" hourly.