Ostia, a city of Latium, at the mouth of the Tiber, on the left bank of its southern arm, 16 m. S. W. of Rome by the Via Ostiensis. It was founded by Ancus Marcius, who established salt works there; and it evidently grew with the growth of Rome, for, though not mentioned again until the second Punic war, it had become then a port and naval station of such importance to Rome that it was one of the two colonies allowed in 207 B. 0. an exemption from military service. During the civil Avar between Sulla and Marius, it was taken and plundered by the latter in 87 B. 0.; hut recovering from this attack, it subsequently became the residence of one of the four quaestors of Italy, with the title of qucastor Ostiensis. But the deposition constantly made by the Tiber gradually filled up its port, and the difficulty of furnishing the city with grain induced the emperor Claudius to construct an artificial harbor on the right bank of the river 2 m. W. of Ostia. This new basin was called Tortus Augusti, and Trajan added an inner basin (Portus Trajani). Despite the rivalry of the town of Portus, which sprung up around the new harbor, Ostia continued to prosper, and contained in its zenith 80,000 inhabitants. But about A. D. 830 it was entirely in ruins.
The modern Ostia is a small town, from which the population, excepting some 200, has been driven away by the pestilential malaria. Although originally founded on the sea, it is now 3 m. distant.