Petronius Maximus (c.396 - 455), was a Roman aristocrat, and briefly Western Roman Emperor for part of 455.

His senatorial career was as follows. His earliest known office was praetor, held c.411; around 415 he served as a tribunus et notarius, which was an entry position to the imperial bureaucracy, and led to his serving as Comes sacrarum largitionum (Count of the Sacred Largess) between 416 and 419, as well as Urban Prefect between the years 419 and 433. In 433 he was consul, in 439 Praetorian Prefect of Italy, and in 443 he was consul a second time. When he was granted the title of Patrician in 445, he was briefly the most honored of all non-Imperial Romans, until the third consulate of Aetius, generalissimo of the Western empire, the following year.

It is clear that the enmity between Maximus and Aetius led to the catastrophic events that brought down the Western Roman Empire. According to the account of John of Antioch, Maximus and the eunuch Heraclius persuaded the emperor Valentinian III to have Aetius killed – which he did by his own hands. The historian John further alleges that Valentinian's death (March 16, 455) at the hands of Optila and Thranstila was also at Maximus' instigation.

Following Valentinian's death, there was no one obvious successor to the throne: Maximus competed with one Maximianus, son of Domninus, who had been a bodyguard of Aetius, and with the future emperor Majorian, who had the backing of the empress Eudoxia. Maximus managed to defeat his rivals by gaining control of the palace and forced Eudoxia to marry him.

Maximus quickly appointed Avitus as Master of Soldiers, and sent him on a mission to Toulouse to gain the support of the Visigoths; however, by the time Avitus arrived, Maximus was dead, and the mission pointless. Within two months of Maximus gaining the throne, word came that Gaiseric, king of the Vandals, had arrived in Italy, news that panicked the inhabitants of Rome. In the disorder Maximus was killed, either by a mob or by "a certain Roman soldier named Ursus".

Three days after Maximus' death on April 22, Gaiseric entered Rome with his army. While the Vandals looted the city and captured people as slaves or hostages, in response to the pleas of Pope Leo I, they desisted from more destructive behavior that accompanied the sack of a city – arson, torture, and murder.