Porto (Italian) or Portus (Latin) was a town in Lazio or Latium, just south of Rome, Italy. It was an ancient harbour on the right bank of the mouth of the Tiber.
Rome's original harbour was Ostia. Claudius constructed the first harbour on the Portus site, 4 km (2.5 mi) north of Ostia, enclosing an area of 69 hectares (170 acres), with two long curving moles projecting into the sea, and an artificial island, bearing a lighthouse, in the centre of the space between them. The foundation of this lighthouse was provided by filling one of the massive Obelisk ships, used to transport an obelisk from Egypt to adorn the spina of Vatican Circus, built during the reign of Caligula. The harbour thus opened directly to the sea on the north-west and communicated with the Tiber by a channel on the south-east. The object was to obtain protection from the prevalent south-west wind, to which the river mouth was exposed. Though Claudius, in the inscription which he caused to be erected in A.D. 46, boasted that he had freed the city of Rome from the danger of inundation, his work was only partially successful: in 62 AD Tacitus speaks of a number of grain ships sinking within the harbour during a violent storm. Nero gave the harbour the name of "Portus Augusti".