The Basilica of Santa Maria in Trastevere vies with Santa Maria Maggiore for the primacy of Christian places of worship in Rome.

According to an ancient tradition, the basilica was built on the site of a military guesthouse and where, in 38 BC, a strange phenomenon occurred: a gush of oil gushed from the floor, an event that was interpreted by Christians as a premonitory sign of the coming of Christ (the Lord's Anointed). At the end of the central nave, the inscription "Fons Olei" indicates the spot where the oil gushed.

It was initially built as a domestic church (domus ecclesiae) in the 3rd century. First Pope Callixtus I and then Pope Julius I (337-352) transformed it into a large basilica dedicated to the worship of the Virgin Mary. In the 12th century, Pope Innocent II rebuilt the basilica using marble from the Baths of Caracalla. His successors, over the centuries, enriched the basilica with decorations.

The façade is striking for its splendid colorful mosaics, created by Roman craftsmen in the 13th century. They depict the Madonna breastfeeding the Child, flanked on either side by two small figures of donors and a sequence of ten female saints holding a lit lamp.

At the bottom of the façade is the entrance portico, designed by Carlo Fontana in 1702, with five arches enclosed by gates. Under the portico are walled-in numerous marble archaeological pieces and a large mosaic by an unknown artist depicting the Madonna breastfeeding the Child.

The interior of the basilica, with three naves separated by twenty-two granite columns, features a Cosmatesque floor (red, green, and gold) by Vespignani. The apse walls feature late 13th-century mosaics by Pietro Cavallini, depicting six scenes from the most important moments in the life of the Mother of God: the Birth of the Virgin, the Annunciation, the Nativity, the Adoration of the Magi, the Presentation in the Temple, and the Death of Mary. Pietro Cavallini's Stories of the Virgin are a true masterpiece: although the influence of Byzantine mosaics is evident, the use of extremely small tesserae allowed Cavallini to create vibrant pictorial effects that were a true innovation for the time. The golden coffered ceiling was created by Domenichino in the 17th century. The carved coffers depict crosses and stars. In the central octagon, the artist himself painted the image of the Assumption of Mary.