Portus was a large artificial harbour of Ancient Rome. Sited on the north bank of the north mouth of the Tiber, on the Tyrrhenian coast, it was established by Claudius and enlarged by Trajan to supplement the nearby port of Ostia.
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 northwest and communicated with the Tiber by a channel on the southeast.
The object was to obtain protection from the prevalent southwest wind, to which the river mouth was exposed. Though Claudius, in the inscription which he caused to be erected in AD 46, boasted that he had freed the city of Rome from the danger of inundation, his work was only partially successful: in AD 62 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".
It was probably Claudius who constructed the new direct road from Rome to Portus, the Via Portuensis which was 24 km (15 mi) long. The Via Portuensis ran over the hills as far as the modern Ponte Galeria, and then straight across the plain. An older road, the Via Campana, ran along the foot of the hills, following the right bank of the Tiber, and passing the grove of the Arval Brothers at the sixth mile, to the Campus salinarum romanarum, the saltmarsh on the right bank from which it derived its name.
In AD 103 Trajan constructed another harbour farther inland—a hexagonal basin enclosing an area of 39 hectares (97 acres), and communicating by canals with the harbour of Claudius, with the Tiber directly, and with the sea, the last now forming the navigable arm of the Tiber (reopened for traffic by Gregory XIII and again by Paul V). It bore the name Fossa trajana, though its origin is undoubtedly due to Claudius. The basin itself is still preserved, and is now a reedy lagoon. It was surrounded by extensive warehouses, remains of which may still be seen: the fineness of the brickwork of which they are built is remarkable.
"Portus was the main port of ancient Rome for more than 500 years and provided a conduit for everything from glass, ceramics, marble and slaves to wild animals caught in Africa and shipped to Rome for spectacles in the Colosseum."
In 2010, "one of the biggest canals ever built by the Romans" was discovered to have been built in Portus, in an ancient port increasingly being seen as important as Carthage or Alexandria. It connected Portus with Ostia and can be seen on the map connected to the Fosse Traiana and pointed south. For some 400 years, from the late second century AD into the fifth and sixth centuries, this 100-yard-wide (90 meter) canal was used to ship goods from all over the Empire to Rome.
By means of these works Portus captured the main share of the harbour traffic of Rome, and though the importance of Ostia did not at once decrease we find Portus already an episcopal see in Constantine's time not very long (if at all) after Ostia, and as the only harbour in the time of the Gothic wars.
Its abandonment dates from the partial silting up of the right arm of the Tiber in the Middle Ages, which restored to Ostia what little traffic was left. To the west of the harbour is the cathedral of Saint Rufina (10th century, but modernized except for the campanile) and the episcopal palace, fortified in the Middle Ages, and containing a number of ancient inscriptions from the site. On the island (Isola Sacra) just opposite is the church of S. Ippolito, built on the site of a Roman building, with a picturesque medieval campanile (13th century ?), as well as the Isola Sacra Necropolis; 3.2 km (2.0 mi) to the west is the modern village of Fiumicino at the mouth of the right arm of the Tiber, which is 34 km (21 mi) westsouthwest by rail from Rome. It is a frazione, or portion of the commune of Rome. 5 km (3.1 mi) to the north is the pumping station by which the lowland (formerly called Stagno di Maccarese, now reclaimed and traversed by many drainage canals) between there and Maccarese is kept drained (Bonifica di Maccarese).
The site can still be fairly clearly traced in the low ground to the east of Fiumicino, and the lighthouse is represented on coins, mosaics, bas-reliefs such as the Torlonia Harbor Relief. The harbour is generally supposed to have been protected by two moles with a breakwater in front, on which stood the lighthouse, with an entrance on each side of it. Trial soundings made in 1907 showed that the course of the right-hand mole is represented by a low sand-hill, while the central breakwater was only some 170 m long, and probably divided from each of the two moles by a channel some 135 m wide. The existence of two entrances is, indeed, in accordance with the evidence of coins and literary tradition, though the position of that on the left is not certain, and it may have been closed in later times. The whole course of the left-hand mole has not yet been traced, but it seems to have protected not only the south-west but also a considerable portion of the north-west side of the harbour.
Many other remains of buildings exist; they were more easily traceable in the 16th century when Pirro Ligorio and Antonio Labacco made plans of the harbour. Considerable excavations were carried on in 1868, but unfortunately with the idea of recovering works of art and antiquities; and the plan and description given by Rodolfo Lanciani (Annali del institute, 1868, 144 sqq.) were made under unfavourable circumstances.
Ostia, just opposite, on the left bank of the Tiber, was increasingly depopulated after Vandal and Saracen attacks. Porto was the main port on the Tyrrhenian Sea until the 6th century AD. Later it decayed, but maintained some importance as the episcopal see which, from 313, was made independent from that in Ostia. Ostia and Porto both were chosen to be amongst the seven suburbicarian dioceses, which are still in existence, and reserved for the members of the highest order of Catholic Cardinals, the Cardinal Bishops, so the prelates of these otherwise insignificant Roman suburbs outrank all archbishops, even the patriarchs.