Marcus Aemilius Aemilianus (c. 210 – September 253), also known as Aemilian, was Roman emperor for three months in 253.

During the reign of Trebonianus Gallus and his son Volusianus (251–253), Aemilian was sent to the Balkans to command an army. His primary responsibility was to assure peace along the Danube frontier, which had been subject to several attacks by the Goths led by king Cniva.

Gallus secured the throne after the death of Emperor Decius at the hands of Cniva in the Battle of Abrittus (251), and later had to manage an outbreak of plague that devastated Rome. He was not popular with the army, mainly due to humiliating treaties signed in 251 with the Goths and King Shapur I of Persia who attacked Syria. According to John of Antioch, upon his appointment to the Moesian command, Aemilian was already envious of Gallus and plotted treachery against him. He was also an opponent of the Roman Senate, and his seditious plans are confirmed by Jerome and Jordanes.

In 253, the Goths, led by king Cniva, claimed they had not received the tribute due from the Romans according to the treaty of 251. They crossed the border and attacked Cappadocia, Pessinus, and Ephesus. Modern historians believe that this missing payment was not a change in Roman policy, and the Goths were more likely trying to capitalize on their military prowess.  Aemilian had command of the army assigned to defend the area, but the recent defeat at the Battle of Abrittus put his troops on edge. Aemilian exhorted them, reminding them of Roman honor and promising tribute from the Goths. The Romans took the Goths by surprise, killing most of them, followed by an invasion of Goth territory resulting in booty and the liberation of prisoners. The Roman soldiers, gathered by Aemilian, acclaimed him emperor.  Jordanes claims, however, that Aemilian's troops plundered Roman territory, rather than keep the tribute of the Goths.

With his few men, Aemilian left his province unguarded and moved quickly towards Rome to meet the legitimate emperor, Gallus, before the latter could receive reinforcements. While Aemilian descended upon Rome along the Flaminian Way, Gallus and Volusianus had him proclaimed "enemy of the State" by the Roman senate, then exited Rome to meet the usurper. This strategy suggests that Aemilian's army was smaller than theirs, as they probably did not expect reinforcements to come in time but trusted their larger army to win the clash.  The two armies met at the Battle of Interamna Nahars near modern Terni, at the southern end of the eastern branch of the Flaminia, and Aemilian won the battle; Gallus and Volusianus fled to the north with a few followers, probably as a delay tactic before the arrival of reinforcements, but, in August 253, at Forum Flaminii (modern San Giovanni Profiamma), on the western branch of the Flaminia, they were killed by some of their own guards, who thought that their betrayal could earn them a reward.

Aemilian continued towards Rome. The Roman senate, after a short opposition,  decided to recognize him as emperor. According to some sources, Aemilian then wrote to the Senate, promising to fight for the Empire in Thrace and against Persia, and to relinquish his power to the Senate, of which he considered himself a general.  Aemilian received the titles of Pius, Felix and Pater Patriae, the tribunicia potestas, and was elevated to the rank of pontifex maximus; he was not, however, elevated to consulate (possibly a hint of his non-senatorial birth).  His coinage shows that his propaganda focused on his capability as a military commander—he defeated the Goths when nobody thought this possible, and thus he was the right man for the job of restoring the power of the Roman Empire.

Valerian, governor of the Rhine provinces, was on his way south with an army which had been called in as a reinforcement by Gallus.  But modern historians believe this army, possibly mobilized for an incumbent campaign in the East, moved only after Gallus' death to support Valerian's bid for power.  Emperor Aemilian's men, fearful of a civil war and Valerian's larger force, mutinied. They killed Aemilian at Spoletium  or at the Sanguinarium bridge, between Oriculum and Narnia (halfway between Spoletium and Rome), and recognized Valerian as the new emperor.  After Aemilian's death, which happened between late July and mid-September, a damnatio memoriae against him was declared.

The life and troubled administration of emperor Aemilian was summarized by the Roman historian Eutropius stating that:

Aemilianus came from an extremely insignificant family, his reign was even more insignificant, and he was slain in the third month.