Constantius III (died 2 September 421) was briefly Western Roman emperor in 421, having earned the throne through his capability as a general under Honorius.

Constantius went on to lead campaigns against various barbarian groups in Hispania and Gaul, recovering much of both for the Western Roman Empire.

[1][2][3][4][5] Constantius served as a general under Honorius, rising to the rank of Magister militum (Master of the Soldiers) by 411.

[6] Constantine refused to surrender, hoping to last until the return of his general Edobichus, who was raising troops in northern Gaul.

[11] Constantius initiated a campaign against the Visigoths in northern Hispania in 416, blockading them in order to starve them and force their submission.

Soon after, the Visigoth king, Wallia, surrendered to Rome, agreeing to return Galla Placidia (the sister of Honorius, who had been captured by Alaric c. 412, and been forced into marriage with Athaulf, who was by this time dead), and to wage war against the Vandals and other barbarians who the Romans were still in conflict with, in exchange for food supplies.

[15] Constantius's position of magister militum and his skill as a commander allowed him to gain huge influence over the Western Roman Empire, comparable to the earlier Stilicho.

Honorius only continued to reign until his death in 423, when Valentinian III, the minor son of Constantius, succeeded him, with Galla Placidia as regent.