The Altar of Our Lady's Immaculate Conception is situated in the north transept. It was installed in 1902 and is made entirely of marble. The tabernacle of this altar contains Holy Relics.
The pair of windows flanking Our Lady's Altar combine into a single work. One part shows Blessed Pope Pius IX holding his definition of the dogma of the Immaculate Conception (Dec.8, 1854). On the other side of the altar, Pope Leo XIII is shown.