The arch was originally located in the eastern part of Forum Boarium, directly above one of the branches of the old sewer system Cloaca Maxima, which had its outlet  into the Tiber. Today it is situated in direct proximity of the Church of San Giorgio in Velabro. It was created in the middle of the IV century and it is a tetrapylon (Quadrifrons) arch, which means it possesses four identical façades with semi-circular arcades. In the interior, it has a cross and ribbed vault.  Its walls built of bricks and concrete, covered with marble come from various older structures, which were obtained during its construction. Decoration of the façades consists of slabs and decorative niches (12 on each side), in which most likely figures of protective deities were located. The only ones that remain until today are the four adorning the four keystones of the arch – definitely depicting two important Roman goddesses – Roma and Minerva and probably Juno and Ceres.

Perhaps, it was a triumphant arch, to which the preserved inscription from the year 357 (Arcus Divi Constantini) seems to point. It perhaps, tells of the founding which took place during the stay of Emperor Constantius II, the son of Constantine the Great, in Rome (at that time the imperial residence was located in Constantinople) and it was to commemorate the divine Constantine – the founding father.

The arch was built at the crossroads of two important communications arteries which intersected here and some researchers believe, that its name comes not from Janus, but from the Latin word ianus, meaning covered passageway, while the arch itself was a type of a currency exchange for the bankers and traders who worked here.

During the Middle Ages the arch was built-over and raised turning it into a fortress of the Frangipani family. On the drawings of Giovanni Battista Piranesi from the XVIII century we can still see the high attic at its top, which was demolished in 1830, under the impression that it came from medieval times. Its parts, as well as the inscription informing about Constantius’s founding are found in the Church of San Giorgio in Velabro.