Ostia, Italy, the port of ancient Rome. Ostia was an important commercial city and port for several centuries. It was situated on the Tyrrhenian Sea at the mouth of the Tiber, 14 miles (23 km) southwest of Rome. Due to constant depositing of silt at the river's mouth, Ostia is now three miles (5 km) inland. It is the site of archeological explorations and is a tourist attraction. At the mouth of the Tiber now is a resort town called Lido di Ostia.

Excavations at Ostia in the 19th and 20th centuries have provided knowledge of how an ancient Roman city was laid out and how the people lived. Remains include a theater (55 B.C.), which is used today to present classical plays; the Capitolium (2 A.D.), a temple; the baths (2 B.C.); a fire brigade station (2 A.D.); the temples of Mithra and Cybele (2 A.D.); and many dwellings, offices, and warehouses.

According to tradition. Ostia was founded in the sixth century B.C by Ancus Martius, the fourth legendary king of Rome. The city developed as a port in the first century B.C., and had a population of 80,000. In the first and second centuries A.D. the emperors Claudius, Nero, and Trajan built and enlarged the artificial harbor at nearby Porto. Loss of trade, plus malaria epidemics, caused the decline of Ostia after the third century. By the sixth century it was almost deserted.