Libius Severus, sometimes enumerated as Severus III, was Western Roman emperor from November 19, 461, to his death on November 14, 465. A native of Lucania, Severus was the fourth of the so-called "Shadow Emperors" who followed the deposition of the Valentinianic dynasty in 455. He ruled for just under four years, attaining the throne after his predecessor, Majorian, was overthrown by his magister militum, Ricimer. Severus was the first of a series of emperors who were highly dependent on the general, and it is often presumed that Ricimer held most of the de facto power during Severus's reign.

Severus's reign was marked by diplomatic tension and an erosion of Rome's control over the non-Italian provinces. Diplomatically, Severus failed to secure the eastern emperor Leo's recognition, and the alliance Majorian had made with Vandal king Gaiseric crumbled as the Vandals raided Italy. In Gaul and Dalmatia officials loyal to Majorian refused to submit to Severus's rule, and Northern Italy was invaded by the Alans.

Severus remains an extremely obscure figure. The ancient sources are almost completely mute on his life and character. Because of the size of Ricimer's influence, no single imperial action can be definitively attributed to Severus; thus the extent of Ricimer's control over imperial affairs during the reign of Severus remains a point of contention among scholars.

Modern scholars are in agreement that Severus's first two names are Libius Severus. However, in Late Latin, the interchanging of b and v became common, and thus Severus's cognomen is sometimes rendered incorrectly as Livius.

In addition to these two names, Severus is sometimes referred to by a third name, Serpentius. The Chronicon Paschale uses only this name for the emperor, and the chronicle of Theophanes the Confessor uses both Severus and Serpentius. Modern scholars are divided as to the authority of this attribution: some (including the PLRE) say the text is corrupt and its meaning uncertain, while others argue it was a signum or supernomen deriving from the word for serpent. Among those who accept the name, its source—whether it is of Eastern or Western origin—is also disputed.

By the time of Severus, the Roman Empire's governance had been firmly split between two centers of power: one in the East centered at Constantinople and one in the West centered at either Rome or Ravenna. During the late fourth and early fifth century the East saw a period of relative peace. The West on the other hand underwent a series of invasions, major political upheavals, and losses of important provinces. By the early 460s, Britain had been abandoned, Africa had been conquered by the Vandals, and Hispania was occupied by the Suebi and the Visigoths (who were foederati of the empire).

During this time, in both the Eastern and Western courts, barbarian generals became increasingly influential; at times, these generals' power rivaled even the emperors'. Of these figures, the most prominent in the West were Stilicho (under the emperor Honorius) and Aëtius (under Valentinian III). After Aëtius's assassination in 454, the western empire entered a downward spiral. Valentinian was deposed, his surviving family was taken captive by Gaiseric, and the replacement emperor (Maximus) was killed while Rome was sacked.

Out of this political chaos, the magister militum Ricimer and general Majorian were able to quickly seize power. Ricimer was a popular and successful military commander, but because he was of non-Roman origin, he was not an acceptable candidate for emperor in the minds of the Senate and people of Rome. Thus, Majorian became emperor with Ricimer still retaining significant political and military authority.