Valerius Licinianus Licinius (Greek: Λικίνιος; c. 265 – 325) was Roman emperor from 308 to 324. For most of his reign he was the colleague and rival of Constantine I, with whom he co-authored the Edict of Milan, AD 313, that granted official toleration to Christians in the Roman Empire. He was finally defeated at the Battle of Chrysopolis (AD 324), and was later executed on the orders of Constantine I.
Born to a Dacian peasant family in Moesia Superior, Licinius accompanied his close childhood friend, the future emperor Galerius, on the Persian expedition in 298. He was trusted enough by Galerius that in 307 he was sent as an envoy to Maxentius in Italy to attempt to reach some agreement about the latter's illegitimate political position. Galerius then trusted the eastern provinces to Licinius when he went to deal with Maxentius personally after the death of Severus II.
Upon his return to the east Galerius elevated Licinius to the rank of Augustus in the West on 11 November 308, and under his immediate command were the Balkan provinces of Illyricum, Thrace and Pannonia. In 310 he took command of the war against the Sarmatians, inflicting a severe defeat on them. On the death of Galerius in May 311, Licinius entered into an agreement with Maximinus Daza to share the eastern provinces between them. By this point, not only was Licinius the official Augustus of the west but he also possessed part of the eastern provinces as well, as the Hellespont and the Bosporus became the dividing line, with Licinius taking the European provinces and Maximinus taking the Asian.
An alliance between Maximinus and Maxentius forced the two remaining emperors to enter into a formal agreement with each other. So in March 313 Licinius married Flavia Julia Constantia, half-sister of Constantine I, at Mediolanum (now Milan); they had a son, Licinius the Younger, in 315.