Julius Nepos (died 9 May 480), or simply Nepos, ruled as Roman emperor of the West from 24 June 474 to 28 August 475. After losing power in Italy, Nepos retreated to his home province of Dalmatia, from which he continued to claim the western imperial title, with recognition from the Eastern Roman Empire, until he was murdered in 480. Though Nepos' successor in Italy, Romulus Augustulus (r. 475–476), is traditionally deemed the last western Roman emperor, Nepos is regarded by some historians as the true last emperor of the west, being the last widely recognised holder of the position.

A native of Dalmatia, Nepos began his career as the semi-autonomous governor of the province, succeeding his uncle Marcellinus, a prominent general, as magister militum ('master of troops') of Dalmatia. After the death of the western emperor Anthemius (r. 467–472), who had been appointed by the eastern emperor Leo I (r. 457–474), as well as Anthemius' successor Olybrius (r. 472), Leo sought to assert his authority in the west, granting Nepos command of an army in December 473 to attack Italy and depose Glycerius (r. 473–474), who had been proclaimed emperor by the Burgundian general Gundobad. Nepos left for Italy in the spring of 474, backed by Leo's successor Zeno, and landed with his army at Portus, near Rome. Nepos swiftly deposed Glycerius and was crowned western emperor in Rome on 24 June 474. He was the last emperor to be crowned in the city until Charlemagne in the ninth century. Whether the original intention of the invasion was to install Nepos as western emperor is unclear, but in any event, he was quickly recognised as the legitimate western emperor by Zeno.

Nepos worked to restore the prestige and authority of the Western Empire, though mostly unsuccessfully. He may have repelled a Visigothic attack on Italy and managed to once more reduce the Burgundians into foederati. Nepos focused most of his attention on reasserting imperial control and authority in Gaul, but the Western Empire could no longer project enough strength to halt Visigothic conquests in the region. The failure to defeat the Visigoths in Gaul, and Zeno's brief overthrow in Constantinople by the usurper Basiliscus, weakened Nepos' already shaky position in Italy. In 475, Nepos' newly appointed magister militum Orestes revolted and marched on Ravenna, capital of the Western Empire. Unable to deal with Orestes' forces, Nepos fled back to Dalmatia and two months later, Orestes proclaimed his young son Romulus Augustulus as emperor.

Although no longer in control of Italy, Nepos never renounced his claim to the Western Empire and continued to be recognised as the legitimate western emperor by the Eastern Empire. In 476, the barbarian general Odoacer deposed Romulus Augustulus and became the first king of Italy. Nepos repeatedly petitioned Zeno, who by then had defeated Basiliscus, for help in regaining control of Italy, though all he achieved was nominal recognition by Odoacer, who minted coins in Nepos' name but otherwise mostly ignored him. In 480, Nepos was murdered by two of his generals, Ovida and Viator, perhaps in Diocletian's Palace, possibly while planning an expedition of his own to recover Italy.

The idea that the Roman Empire had grown too large to efficiently be managed by one emperor, and was more appropriately governed by two co-ruling emperors, had become established by the time of emperor Diocletian (r. 284–305) in the late 3rd century. Throughout the 4th century, various different divisions were made until the empire was firmly and permanently divided into western and eastern spheres of imperial administration from the death of emperor Theodosius I (r. 379–395) in 395. Though modern historians typically use the terms Western Roman Empire and Eastern Roman Empire to describe the new political situation, the Romans themselves never considered the empire to have been formally divided, still viewing it as a single unit, although most often having two rulers rather than one. The Western Roman Empire was more rural than the east, with fewer people and a less stable economy. Throughout the 5th century, it experienced an increasing number of Germanic barbarian invasions and settlements and a period of decline. In 410, the Visigoths under Alaric I had sacked Rome; in 455, the last western emperor of the Theodosian dynasty, Valentinian III (r. 425–455), was deposed and murdered. That same year, Rome was sacked again, this time by the Vandals under Gaiseric, who captured Valentinian's widow, Licinia Eudoxia, and two of his daughters, Eudocia and Placidia. The Western Roman army gradually became increasingly reliant on barbarian mercenaries. After Valentinian's murder, the most powerful barbarian generals, such as Ricimer, became politically dominant, ruling through proclaiming puppet emperors. In the time between the death of Valentinian and the accession of Julius Nepos, a period of less than twenty years, seven different emperors ruled the west and effective imperial control was only exercised in Italy, Raetia and some regions of Gaul.

Julius Nepos was a native of the Roman province of Dalmatia. Dalmatia, although politically, economically and geographically oriented towards the Western Roman Empire, had formally been under the authority of the Eastern Roman Empire since 437, when the western emperor Valentinian III had ceded it to the east. In practical terms, the province was often more or less autonomous. Nepos was the son of Nepotianus (died 465), a general who served under the western emperor Majorian (r. 457–461). The nephew of the magister militum ('master of troops', a high-ranking military officer) Marcellinus (died 468), being the son of one of Marcellinus' sisters. Nepos may have been part of a prominent local Roman family in Dalmatia. This is supported by memorial inscriptions from four roughly contemporary individuals; Aelia Nepotes, Aelia Nepos, another Julius Nepos and Nepotes, having been identified from Dalmatia.

Nepos' uncle Marcellinus was a prominent late Roman general, having fought the Vandals in several campaigns and playing a notable role in momentarily fending off Vandal attempts at controlling Sardinia. In 461, after the murder of Emperor Majorian by Ricimer, Marcellinus appears to have planned to invade Italy to depose Ricimer's puppet emperor Libius Severus (r. 461–465), but he was dissuaded from this course of action by the eastern emperor Leo I. Nepos married a Roman noblewoman whose name is not recorded, but who was related to the eastern empress Verina, possibly a niece of Leo I.

By the time of his death, Marcellinus had been a semi-autonomous governor in Dalmatia, and after his death, Nepos inherited this position. Nepos is first attested on 1 June 473, when he is described as magister militum Dalmatiae, although it is possible that he had held that rank ever since Marcellinus' death. Nepos might have held this position until he became emperor in 474, by which point he had also become a patrician.