The Porta Maggiore Basilica is an underground basilica discovered in 1917 near Porta Maggiore in Rome. It is dated to the first century BC. It is believed to have been the meeting place of the neo-Pythagoreans, and is the only historical site that has been associated with the neo-Pythagorean movement. This school of mystical Hellenistic philosophy preached asceticism and was based on the works of Pythagoras and Plato. It was a precursor to the basilicas built during the Christian period, centuries later. It was opened to small groups of visitors in April 2015.

In Ancient Roman architecture, a basilica was a large public building with multiple functions that was typically built alongside the town's forum. The basilica was in the Latin West equivalent to a stoa in the Greek East. The building gave its name to the basilica architectural form.