Constantine's Arch, which stands next to the Colosseum, was erected by the Senate and the Roman people at the outer limit of the Forum, in the Via Sacra, in commemoration of the victory achieved over Maxentius at the Milvian Bridge in 312 AD. Before the battle Constantine is reported to have seen a sign, and as a result the Edict of Milan (313 AD) guaranteed freedom of worship for Christians and also restored the properties confiscated from them during the persecutions.

The Arch is in the same architectural style as that of other arches, notably the arch of Septimius Severus, which stands in the Roman Forum and dates from the year 203 AD. However, Constantine's is of better quality: the proportions are more harmoniously conceived and the arrangement of the different decorative parts does not produce the sensation of clutter that can be felt with Septimius' arch.

Many of its decorative elements come from previous monuments. The eight medallions placed above the minor entrances in the arch belong to the time of Hadrian; the eight bas-reliefs on the long sides of the attic (the section above the cornice) date from the reign of Marcus Aurelius; the figures of barbarians over the column architraves and the bas-reliefs on the narrow sides of the attic and on the main arch walls are from the era of Trajan.

The long narrow friezes are from Constantine's own time: they represent the battle of the Milvian Bridge, the siege of Verona and the Emperor's triumphal procession distributing presents to the people.

It is interesting to note the ambiguous mention of 'the divinity' in the dedicatory inscription: it no longer mentions 'the gods' since the magistrates, who were pagan, knew that Constantine favoured the Christians, who worshipped one God only. However, they still attempted to associate Constantine with the oldest pagan religion by depicting him offering a sacrifice.