Magnus Decentius (died 18 August 353)[2] was caesar of the Western Roman Empire from 350 to 353, under his brother Magnentius.

[3] Magnentius usurped power from Constans on 18 January 350, and elevated Decentius as caesar later that year, perhaps in July or August.

The Alamanni began to invade the province, perhaps at the instigation of the emperor Constantius II in order to increase pressure on the usurper.

[a] Decentius, who led his brother's forces in the north, was defeated in a pitched battle by the Alemannic chief Chnodomar, and besieged in Sens.

[6] There news reached him of Constantius' victory at the Battle of Mons Seleucus, and the subsequent suicide of Magnentius.