The Arch of Janus is a tetrapylon triumphal arch, also known as a quadrifron. It's the only remaining structure of its kind in Rome.

It's located at the edges of the Forum Boarium, a market area of ancient Rome. The arch was constructed during the 4th-century using materials from older buildings in the city. It's known as the Arch of Janus but has nothing to do with the Roman god of gates. The arch was never given this name during antiquity. The name likely came from the Latin word ianus during the Renaissance period and derived from its four-fronted, four-arched configuration. 

Many researchers believe the arch was more than likely dedicated to Emperor Constantine or his son Constantius II. Others think that it's not an actual triumphal arch, but rather a shelter for traders who worked the Forum Boarium market.

Forty-eight niches are located throughout the arch and were probably once filled with statues. During the Middle Ages, the arch was transformed into a fortress until 1830, when the medieval additions were removed and the original structure was restored. Despite these efforts, the attic and other portions of the top of the structure were erroneously removed. They were thought to have been non-original additions to the arch.

Since 1993, when a bomb exploded in front of a nearby church, the inside of the arch cannot be accessed.