Marcus Aurelius Valerius Maxentius (c. 283 – 28 October 312) was a Roman emperor from 306 until his death in 312. Despite ruling in Italy and North Africa, and having the recognition of the Senate in Rome, he was not recognized as a legitimate emperor by his fellow emperors.

He was the son of former Emperor Maximian and the son-in-law of Emperor Galerius. The latter part of his reign was preoccupied with civil war, allying with Maximinus against Licinius and Constantine. The latter defeated him at the Battle of the Milvian Bridge in 312, where Maxentius, with his army in flight, purportedly perished by drowning in the Tiber river.

The armies of Maxentius and Constantine met north of the city, some distance outside the walls, beyond the Tiber river on the Via Flaminia. Christian tradition, especially Lactantius and Eusebius of Caesarea, claims that Constantine fought under the labarum in that battle, revealed to him in a dream. Of the battle itself, not much is known – Constantine's forces defeated Maxentius's troops, who retreated to the Tiber, and in the chaos of the fleeing army trying to cross the river, Maxentius fell into the water and drowned. His body was found the next day and paraded through the city, and later sent to Africa, as a sign that he had surely perished. Maxentius' entire family, including his two orphaned children, were also executed.

After Constantine's victory, Maxentius was systematically vilified and presented as a cruel, bloodthirsty and incompetent tyrant.

Maxentius was the last emperor to permanently reside in Rome. He attempted to embellish, restore and improve the ancient capital, carrying out important building works, including the Temple of the Divine Romulus (dedicated to his deceased son), the Basilica of Maxentius, which was completed by Constantine, the villa and the circus of Maxentius.

Maxentius was a prolific builder, whose achievements were overshadowed by Constantine's issue of a damnatio memoriae against him. Many buildings in Rome that are commonly associated with Constantine, such as the great basilica in the forum Romanum, were in fact built by Maxentius.