Old St. Peter's Basilica was the building that stood, from the 4th to 16th centuries, on the spot where the new
St. Peter's Basilica stands today in
Vatican City. Construction of the basilica, built over the historical site of the
Circus of Nero, began during the reign of Emperor
Constantine I. The name "old St. Peter's Basilica" has been used since the construction of the current basilica to distinguish the two buildings.