Constantius II (Latin: Flavius Julius Constantius Augustus;[1][2] 7 August 317 – 3 November 361) was Roman Emperor from 337 to 361. The second son of Constantine I and Fausta, he ascended to the throne with his brothers Constantine II and Constans upon their father's death.

In 340, Constantius' brothers clashed over the western provinces of the empire. The resulting conflict left Constantine II dead and Constans as ruler of the west until he was overthrown and assassinated in 350 by the usurper Magnentius. Unwilling to accept Magnentius as co-ruler, Constantius defeated him at the battles of Mursa Major and Mons Seleucus. Magnentius committed suicide after the latter battle, leaving Constantius as sole ruler of the empire.

His subsequent military campaigns against Germanic tribes were successful: he defeated the Alamanni in 354 and campaigned across the Danube against the Quadi and Sarmatians in 357. In contrast, the war in the east against the Sassanids continued with mixed results.

In 351, due to the difficulty of managing the empire alone, Constantius elevated his cousin Constantius Gallus to the subordinate rank of Caesar, but had him executed three years later after receiving scathing reports of his violent and corrupt nature. Shortly thereafter, in 355, Constantius promoted his last surviving cousin, Gallus' younger half-brother, Julian, to the rank of Caesar.

However, Julian claimed the rank of Augustus in 360, leading to war between the two. Ultimately, no battle was fought as Constantius became ill and died late in 361, though not before naming Julian as his successor.