A team, headed by the University of Southampton, excavated the remains of a building more than 150 metres long, 60 metres wide and five storeys high at Portus, which was the ancient port for Rome.
The structure from about 117 AD, in the reign of Trajan, was used to either build or service ships that travelled across the empire 2,000 years ago to keep Rome supplied with food and goods.
It is largest find of its kind in Italy or the Mediterranean.
It was found close to a distinctive existing hexagonal basin or “harbour” at the centre of the huge ancient port complex, which covers two miles square.
It sits by the side of Fiumicino airport and is now more than two miles from the Mediterranean.
The building is so grand archaeologists think it had some form of imperial connection and might have been used for a base for galleys that transported emperors, such as Hadrian, across the empire on their way to places like Britain.
The latest discovery comes after the team found an ornate private amphitheatre at the same site two years ago. The new building is further proof of a link to the leaders of the Roman world and the importance of the ruins.
University of Southampton professor and Portus project director, Simon Keay, said: “Few Roman imperial shipyards have been discovered and, if our identification is correct, this would be the largest of its kind in Italy or the Med.”