Documents cite the transfer in 898 of the relics of Saint Agapitus, since moved to the interior of the Basilica cimiteriale extramoenia delle Quadrelle.
The original paleo-Christian church was enlarged by bishop Conone with the addition to two lateral aisles and the construction of a crypt in the presbytery.
During the civil conflicts in 1437 between the Colonna family and the papacy, part of the facade including the bell-tower were razed, and the relics of Sain Agapitus were take to what is now Tarquinia.
A century later, they were returned at the request of Cardinal Marcantonio Colonna and pope Sixtus V. They are housed in a bejeweled bust of silver, and displayed in a public procession in honor of the saint held every 18 August.
Among the altarpieces are one depicting the Decapitation of Sant’Agapito by Carlo Saraceni (last chapel on right), a Crucifixion with the Virgin and St Lawrence and a Savior by Girolamo Siciolante da Sermoneta.