Ostia (/ˈɒstiə/, Italian: [ˈɔstja]), officially Lido di Ostia, is a large neighbourhood in the Municipio X of the comune of Rome, Italy, near the ancient port of Rome, which is now a major archaeological site known as Ostia Antica. Ostia is also the only municipio or district of Rome on the Tyrrhenian Sea, and many Romans spend the summer holidays there. It is entirely situated within the municipality of Rome and is the city's seaside resort.

Ostia was the port city of ancient Rome; it had a strategic function for trade, especially for the supply of grain, and as the main base of the Roman navy. It had a fundamental function during the Punic Wars, and after the final destruction of Carthage, and the end of Macedon's independence; by the latter half of the 2nd century BC, Roman control over all of what was later to be dubbed Mare Nostrum ("our sea") had been established.

The ancient city of Ostia stood in a strategic point near the mouth of the Tiber and the sea, which at the time was closer to the city. The modern seaside resort, Lido di Ostia, is in fact located about 5 km (3 miles) south-west of the ancient city and about 28 km from the center of Rome (17.4 miles).

The name of Ostia is derived from the Latin ostia, in turn plural of the neuter ostium i.e. 'mouth', or alternatively 'door, entrance'. The current placename is a reference to the original settlement in the eighth century BC at the mouth of the river Tiber and precisely called Ostia Tiberina.

The settlement of Ostia, according to tradition, was built under the reign of the fourth king of Rome Ancus Marcius, around 630 BC, as the first official Roman colonia on the then coastline of the Tyrrhenian Sea and near the mouth of the Tiber river. Ostia Antica was the port city of ancient Rome, and is often referenced in writings from the times of the Roman Republic and the Roman Empire. It was the main port of Republican Rome; the main function of Ostia was that of supplying salt and grain although the archaeological site suggests that it had, in the beginning, an important military function as the base of the first Roman fleets. Proof of its importance is the exemption from military conscription during the Second Punic War. Later the port, partially blocked by a sandbank, became unsuitable for large ships and was replaced by the hexagonal port of Trajan.

Already since the 3rd century it was the seat of the bishop of Ostia. Ostia Antica was especially important as the site of the death of Saint Monica (mother of Saint Augustine) in 387 in a house property of the Diocesi of Rome, on their way back to Africa after Augustine's conversion to Christianity.

Given the important position at the mouth of the Tiber, Cardinal Giuliano della Rovere ordered the construction of a fortress to defend the river to stem pirate raids; however, a major flood in 1557 diverted the course of the river, nullifying the strategic function of the fort, which was supplanted by Tor San Michele and Tor Boacciana.

An important stand in Italian nationalism of the 19th century was a veneration for the glories of the Roman past, manifested in a wish to revive or recreate various places and institutions connected with Ancient Rome. In 1871, the city of Rome became the capital of the new Kingdom of Italy. A recreation of Ostia was an obvious additional step.