Ancient quadrifrons arch · Early 4th century AD · Rome, Lazio

Arch of Janus (Arcus Divi Constantini)

The Arch of Janus is the only surviving ancient quadrifrons triumphal arch in Rome, standing at a former crossroads in the Forum Boarium — the ancient cattle market — adjacent to the church of San Giorgio in Velabro at 41.8893° N, 12.4828° E. Built in the early 4th century AD, probably during the reign of Constantine I or Constantius II, the marble-clad arch rises 16 metres on four equal faces, each pierced by an arched passage, giving it a distinctive cross-shaped plan suited to marking a junction of roads. Its 48 niches — arranged in two rows of three on each pier — once held statues, and the arch spans the Cloaca Maxima, Rome’s great ancient sewer, at one of the oldest occupied points of the city.

At a glance

Type: Quadrifrons triumphal arch
Period: Early 4th century AD (Constantinian)
Style: Late Roman Imperial, constructed with spolia
Location: Forum Boarium / Velabrum, Rome · 41.8893° N, 12.4828° E

Overview

The Arch of Janus occupies one of Rome’s oldest and most historically layered spaces — the Forum Boarium, the ancient river port and cattle market that was among the first areas of settled habitation along the Tiber. Its modern name derives from the four-faced Roman god Janus and was likely assigned during the Renaissance; the original dedicatory inscription does not survive intact, leaving its precise honorand a matter of scholarly debate. The arch is unique among Rome’s ancient monuments as the sole surviving example of the quadrifrons form.

History

The arch was constructed in the early 4th century AD using spolia — reused marble, brick, and decorative elements salvaged from earlier Roman buildings — a common technique in the late Imperial period when cutting fresh stone was less economical than repurposing existing material. Scholars propose it functioned as a boundary marker, a shelter for traders in the Forum Boarium, or a monument honouring Constantine I or Constantius II. During the Middle Ages the Frangipane family, one of Rome’s most powerful baronial clans, converted the arch into a fortified tower. This medieval reuse paradoxically preserved the structure — though its attic storey was removed in 1830 during clearance works, reducing its original height.

What you see

The arch presents four identical faces of white marble, each centred on a large arched passage flanked by pilasters and decorated with two rows of three empty niches — originally holding portrait statues, none of which have been identified. The keystone of the central north arch bears a relief possibly depicting Minerva or the Palladium, one of Rome’s sacred talismans. The overall structure measures 16 metres in height and 12 metres in width; close inspection of the masonry reveals the patchwork of spolia and earlier building materials from which it is assembled. The adjacent church of San Giorgio in Velabro, damaged by a 1993 Mafia car bomb, provides a poignant counterpoint to the ancient monument.

Cultural significance

As the sole surviving example of the quadrifrons arch type — a form used to mark ancient crossroads and boundaries — the Arch of Janus is an irreplaceable document of late Roman urban planning and triumphal architecture. Its location above the Cloaca Maxima and within the ancient Forum Boarium situates it at the intersection of Rome’s oldest hydraulic, commercial, and sacred geographies.

Practical information

Address: Via del Velabro, 00186 Roma RM (Forum Boarium / Velabrum area)
Hours: Exterior visible at all times; interior has been inaccessible since 1993
Admission: Free (exterior viewing)
Notes: The arch has been closed to public access since a 1993 car-bomb attack targeted the nearby San Giorgio in Velabro church; restoration of both structures has been intermittent

Getting there

The arch is located in the Velabrum, between the Capitoline Hill, the Circus Maximus, and the Tiber. The nearest Metro station is Circo Massimo (Line B), approximately 10 minutes on foot. Bus lines 23, 44, 63, 81, 160, 170, and 280 stop near the Circus Maximus or the Lungotevere. The arch is an easy addition to a walking route between the Palatine/Circus Maximus and the Capitoline Hill.

Sources & resources

Historical events at this place (1)

2023 Visita guidata all'Arcus Divi Costantini

Arch of Janus is the sixteenth-century name of a triumphal arch in the form of a tetrapylon erected in Rome, in the Constantinian era, whose original name was &quot;Arcus Divi Costantini&quot;.



The monument stands on the edge of the Foro Boario, near the church of San Giorgio in Velabro, not far from the Temple of Hercules and the Temple of Portuno. It is one of the rare examples of a tetrastyle preserved to this day. The only other example in Italy is the Arco di Malborghetto, which however was heavily modified in a later period.



It is built largely with marble and materials taken from other previous monuments, towards the middle of the 4th century, therefore with a genesis similar to that of the Arch of Constantine. It must probably be identified with the Arcus Divi Constantini mentioned in the regional catalogs at the Velabrum. The modern name does not refer to the two-faced god Janus, but rather derives from the Latin term ianus, meaning a covered passageway, or doorway.



As the iani witnessed by the sources in the Roman Forum, it was not a triumphal arch, but probably a structure intended for the bankers who worked in the Forum Boarium. The building, which has a square plan and is 12 m high with 16 m on each side, has four massive pillars which support a cross vault. They are built in cement and covered with reused blocks of marble.



Above it must have had an attic floor and a roof perhaps in the shape of a pyramid, whose structure in brick work, which originally must have also been covered in marble, was demolished in 1827 because it was wrongly considered part of the medieval fortification planted above the building Roman by the Frangipane (who had also closed the arches). In the pillars there are many niches which perhaps housed almost life-size statues, there are 12 on each east and west face and 2 on each north and south face and another 10 are fake. In total then 28.



The niches originally had to house statues and were framed by aedicules with small columns, now lost, resting on the cornices, and were covered by a shell-shaped semi-dome carved in the marble blocks of the facing. The only preserved remains of the sculptural decoration are represented by the four female figures on the keystones: the goddess Roma on the eastern side and Minerva on the northern side can be recognized with certainty, while the identification of the other two figures as Juno and Ceres presents greater uncertainties.



The Arch, in addition to having monumental functions, served as a shelter from the inclemency of the weather for the Roman cattle merchants and the money lenders who crowded the Forum. During the Middle Ages, the Roman Frangipane family used it as a fortress, closing its arches; when these works were eliminated, in 1830, the original attic and crowning were also lost, because they were not recognized as works belonging to the original structure.

https://culturalheritageonline.com/places/arch-of-janus-arcus-divi-constantini-l5112/

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Arch of Janus – Arcus Divi Constantini