Situated on the Velia hill, the basilica had a rectangular layout divided into three naves, with a side entrance hall. The wide nave ended with an apse and was covered by three large cross vaults resting on eight Corinthian monolithic columns of Proconnesian marble. The two side naves were divided into three compartments linked with each other and with the entrance hall, covered with large coffered barrel vaults and open with arches towards the interior. The only preserved nave (north aisle), has an apse in the central room decorated with niches framed by columns on corbels, formerly housing statues.
The basilica, built by Maxentius (306-312) in 308 and finished, with some modifications, by Constantine (306-337), partially stood on the pre-existing structures of the Horrea Piperataria, commercial pepper and spice warehouses. The Constantinian intervention was characterized by the laying, in one of the apses, of a colossal statue of the emperor whose imposing fragments can be seen today in the courtyard of the Palazzo dei Conservatori on the Capitoline Hill.