The Theatre of Marcellus is an ancient open-air theatre in Rome, Italy, built in the closing years of the Roman Republic. At the theatre, locals and visitors alike were able to watch performances of drama and song. Today its ancient edifice in the rione of Sant'Angelo, Rome, once again provides one of the city's many popular spectacles or tourist sites.
Space for the theatre was cleared by Julius Caesar, who was murdered before its construction could begin; the theatre was advanced enough by 17 BC that part of the celebration of the ludi saeculares took place within the theatre; it was completed in 13 BC and formally inaugurated in 12 BC by Augustus, named after his nephew Marcus Claudius Marcellus who had died in 23 BC.
The theatre was 111 m in diameter and was the largest and most important theatre in Ancient Rome; it could originally hold between 11,000 and 20,000 spectators. A catalogue complied at the end of the 4th century recorded that the theatre's seating capacity was 17,580 persons. It was an impressive example of what was to become one of the most pervasive urban architectural forms of the Roman world.