Flavius Aetius (also spelled Aëtius; Latin: [aːˈɛtiʊs]; c. 390 – 21 September 454) was a Roman general and statesman of the closing period of the Western Roman Empire. He was a military commander and the most influential man in the Empire for two decades (433–454). He managed policy in regard to the attacks of barbarian federates settled throughout the West. Notably, he mustered a large Roman and allied (foederati) army in the Battle of the Catalaunian Plains, ending an invasion of Gaul by Attila in 451, though the Hun and his subjugated allies still managed to invade Italy the following year, an incursion best remembered for the Sack of Aquileia and the intercession of Pope Leo I. In 454, he was assassinated by the emperor Valentinian III.

Aetius has often been called the "Last of the Romans". Edward Gibbon refers to him as "the man universally celebrated as the terror of Barbarians and the support of the Republic" for his victory at the Catalaunian Plains. J.B. Bury notes, "That he was the one prop and stay of the Western Empire during his life time was the unanimous verdict of his contemporaries."

Aetius was born at Durostorum in Moesia Secunda (modern Silistra, Bulgaria), roughly around 390, as he was described as a "young adolescent" in 405. His father, Gaudentius, was a Roman general and described as a native of the province of Scythia. Aetius' mother, whose name is unknown, was a wealthy aristocratic woman of ancestry from Rome or some other city in the Italian peninsula. Before 425 Aetius married the daughter of Carpilio, who gave him a son, also named Carpilio. Later he married Pelagia, widow of Bonifacius, from whom he had a son, Gaudentius. It is possible that he also had a daughter, whose husband, Thraustila, avenged Aetius' death by killing emperor Valentinian III.

As a boy, Aetius was at the service of the imperial court, enrolled in the military unit of the Protectores Domestici and then elevated to the position of tribunus praetorianus partis militaris, setting him up for future political eligibility. Between 405 and 408 he was kept as hostage at the court of Alaric I, king of the Visigoths. In 408 Alaric asked to keep Aetius as a hostage, but was refused, as Aetius was sent to the court of Uldin, king of the Huns, where he would stay throughout much of the reign of Charaton, Uldin's successor. Some modern historians have suggested that Aetius's upbringing amongst militaristic peoples gave him a martial vigour not common in contemporary Roman generals.

In 423 the Western Emperor Honorius died. The most influential man in the West, Castinus, chose as his successor Joannes, a high-ranking officer. Joannes was not a member of the Theodosian dynasty so he was not recognized by the eastern court. The Eastern Emperor Theodosius II organised a military expedition westward, led by Ardaburius and his son Aspar, to put his cousin, the young Valentinian III (who was a nephew of Honorius), on the western throne. Aetius entered the service of the usurper as cura palatii and was sent by Joannes to ask the Huns for assistance. Joannes lacked a strong army and fortified himself in his capital, Ravenna, where he was killed in the summer of 425. Shortly afterwards, Aetius returned to Italy with a large force of Huns to find that power in the west was now in the hands of Valentinian III and his mother Galla Placidia. After fighting against Aspar's army, Aetius managed to compromise with Galla Placidia. He sent back his army of Huns and in return obtained the rank of comes et magister militum per Gallias, the commander in chief of the Roman army in Gaul.

In 426, Aetius arrived in southern Gaul and took command of the field army. At that time Arelate, an important city in Narbonensis near the mouth of the Rhone, was under siege from the Visigoths, led by their king Theodoric I. Aetius defeated Theodoric, lifted the siege of Arelate, and drove the Visigoths back to their holdings in Aquitania.

In 428, he fought the Salian Franks, defeating their king Chlodio and recovering some territory they had occupied along the Rhine. In 429 he was elevated to the rank of magister militum; this was probably the junior of the two offices of comes et magister utriusque militiae, as the senior is known to have been the patrician Flavius Constantinus Felix, the most influential man in those years, and a supporter of Galla Placidia.

Begin 430 the Goths led by Anaolsus attacked Arelate again but were defeated by Aetius.