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.

Maxentius was the last emperor permanently to 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 the son of Emperor Maximian and his Syrian wife Eutropia. As his father became emperor in 285, he was regarded as the crown prince who would eventually follow his father on the throne. He seems not to have served, however, in any important military or administrative position during the reign of Diocletian and his father. The exact date of his marriage to Valeria Maximilla, daughter of Galerius, is unknown. He had two sons, Valerius Romulus and an unknown one.

In 305, Diocletian and Maximian abdicated, and the former Caesares Constantius and Galerius became Augusti. Although two sons of emperors – Constantine and Maxentius – were available, they were passed over for the new tetrarchy, and Valerius Severus and Maximinus Daza were appointed Caesars. Lactantius' Epitome states that Galerius hated Maxentius and used his influence with Diocletian to see that Maxentius was ignored in the succession; perhaps Diocletian also thought Maxentius was not qualified for the military duties of the imperial office. Maxentius retired to an estate some miles from Rome.

When Constantius died in 306, his son Constantine was crowned emperor on July 25 and subsequently accepted by Galerius into the Tetrarchy as Caesar. This set the precedent for Maxentius' accession later in the same year.

When rumours reached the capital that the emperors tried to subject the Roman population to the capitation tax, like every other city of the empire, and wanted to dissolve the remnants of the Praetorian Guard which were still stationed at Rome, riots broke out. A group of officers of the city's garrisons (Zosimus calls them Marcellianus, Marcellus and Lucianus) turned to Maxentius to accept the imperial purple, probably judging that the official recognition which was granted to Constantine would not be withheld from Maxentius, son of an emperor as well. Maxentius accepted the honour, promised donations to the city's troops, and was publicly acclaimed emperor on 28 October 306. The usurpation obviously went largely without bloodshed (Zosimus names only one victim); the prefect of Rome went over to Maxentius and retained his office. Apparently the conspirators turned to Maximian as well, who had retired to a palace in Lucania, but he declined to resume power for the time being.

Maxentius managed to be recognized as emperor in Central Italy and Southern Italy, the islands of Corsica and Sardinia and Sicily, and the African provinces. Northern Italy remained under the control of the western Augustus Valerius Severus, who resided in Mediolanum (Milan).