The partially dug complex, dating back to the 2nd century A.D., is believed to have been funded by Emperor Hadrian as a school to promote liberal arts and culture.
Known as the "Athenaeum" and named after the city of Athens, which was considered the center of culture at the time, the auditorium could accommodate up to 200 people, experts said.
"Hadrian, who was a cultured emperor, wanted to re-establish the tradition of public recitation, conferences and poetry contests, as it used to happen in classic Greece," Roberto Egidi, an archaeologist overseeing the digs, said during a tour.
Egidi said the identification of the auditorium as Hadrian's is "a likely hypothesis" due to the building's specific structure, as well as references in ancient texts. The digs have turned up two terraced staircases used for seating, a corridor and marbled floors, Egidi said.
Egidi also said the building's upper floors are believed to have crumbled during an earthquake.
The auditorium was discovered during excavations at Piazza Venezia, a busy intersection in the heart of Rome, just a few meters (yards) from the Roman Forum.
Archaeologists have been probing the depths of the Eternal City for months to pave the way for some of the 30 stations of the city's planned third subway line. Many of the digs are near famous monuments or on key thoroughfares and several archaeological remains - including Roman taverns and 16th-century palace foundations - have already turned up at Piazza Venezia.
Francesco Giro, a top official with Italy's culture ministry, said the entrance to the subway would be close to the auditorium, but in an area where digs turned up only ancient sewers.
The archaeological investigations are needed only for the subway's stairwells and air ducts, because the 15 miles of subway stations and tunnels will be dug at a depth of 80 to 100 feet - below the level of any past human habitation.
However, most of the digs still have yet to reach levels that date back to Roman times, where plenty of surprises may be waiting.
Rome's 2.8 million inhabitants rely on just two subway lines, which only skirt the city center, leaving it clogged with traffic and tourists.
Plans for a third line that would serve the history-rich heart of Rome have been put off for decades amid funding shortages and fears that a wealth of archaeological discoveries would halt work.