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Constantius III
Emperor of the Western Roman Empire
Constantius on a solidus. The reverse shows Constantius as a general, holding Victory in one hand and a captive enemy in the other.
Reign	421 (7 months, as co-emperor in the west with Honorius)
Full name	Flavius Constantius
Born	Naissus (modern-day Niš)
Died	2 September 421
Predecessor	Honorius (alone)
Successor	Honorius (alone)

Flavius Constantius (d. 2 September 421), whose name is traditionally anglicized as Constantius III, was a late Roman general, politician, and emperor. He was the power behind the throne for much of the 410s, and in 421 briefly became co-emperor of the Western Empire with Honorius.

Constantius was born in Naissus (modern-day Niš) and was probably a career soldier. As a magister militum under Honorius, he gained note by his successful campaigns in defense of the Western Roman Empire, in which he pushed back barbarian invasions and ended the revolt of the usurper Constantine III. As a result, he was given the title of Patrician, and began to exert more and more influence over the weak Honorius. In 417 he married Honorius' sister, Galla Placidia, and on February 8, 421, was elevated to co-Emperor. At this point, he effectively ruled the West. Notably, Constantius reportedly complained about the loss of personal freedom and privacy that came with the imperial office.

Honorius' nephew, the Eastern Roman emperor Theodosius II refused to recognize Constantius' imperial status. Constantius reportedly intended to launch a campaign against the Eastern Empire to force recognition of his rights, but before anything could come of these plans, he died suddenly on September 2 after less than seven months as emperor.

Constantius and Galla Placida had two children, the future emperor Valentinian III and Justa Grata Honoria.

Constantius' success in rising from head of the dwindling Roman army to Imperial rank obviously influenced the actions of later holders of the patrician office, a list that includes Aëtius and Ricimer; however, only Petronius Maximus would ultimately make the same leap, and his reign would prove to be even shorter than Constantius'.

• Media on Constantius III in the Wikicommons.