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Magnus Decentius (died 18 August 353) was a usurper of the western portion of the Roman Empire against emperor Constantius II. Decentius was the brother of Magnentius, who had revolted against Constantius on 18 January 350.
Magnentius elevated Decentius as Caesar or Augustus in winter of 351-52, to oversee the defence of Gaul and the Rhine frontier. He was appointed consul ordinarius in 352. In the following year, after he had lost the battle of Mursa Major, Magnentius' exactions to finance the war drove Gaul into revolt against his dictatorial rule, and Decentius was expelled from the capital, Treves, which headed the revolt.
Constantius had meantime incited the Alemans to invade the province in order to increase the pressure on the usurper. Decentius, who led his brother's forces in the north, was defeated in a pitched battle by the Alemanic chief Chnodomar, and besieged in Sens. Decentius, when he heard of Magnentius' defeat by Constantius at the Battle of Mons Seleucus and his subsequent suicide, strangled himself in the besieged city, signalling the end of the civil war.