Constantine I (27 February 272 – 22 May 337), also known as Constantine the Great, was Roman emperor from AD 306 to 337 and the first Roman emperor to convert to Christianity. He played a pivotal role in elevating the status of Christianity in Rome, the Edict of Milan decriminalising Christian practice and ceasing Christian persecution. This was a turning point in the Christianisation of the Roman Empire. He founded the city of Constantinople (now Istanbul) and made it the capital of the Empire, which it remained for over a millennium.

Born in Naissus, a city located in the province of Moesia Superior (now Niš, Serbia), Constantine was the son of Flavius Constantius, a Roman army officer from Moesia Superior, who would become one of the four emperors of the Tetrarchy. His mother, Helena, was a woman of low birth, probably from Bithynia. Later canonised as a saint, she is credited for the conversion of her son in some traditions, though others believe that Constantine converted her. He served with distinction under emperors Diocletian and Galerius. He began his career by campaigning in the eastern provinces against the Persians, before being recalled to the west in AD 305 to fight with his father in the province of Britannia. After his father's death in 306, Constantine was proclaimed as augustus (emperor) by his army at Eboracum (York, England). He eventually emerged victorious in Civil wars of the Tetrarchy against the emperors Maxentius and Licinius to become the sole ruler of the Roman Empire by 324.

Upon his accession, Constantine enacted many reforms to strengthen the empire. He restructured the government, separating civil and military authorities. To combat inflation, he introduced the solidus, a new gold coin that became the standard for Byzantine and European currencies for more than a thousand years. The Roman army was reorganised to consist of mobile units (comitatenses), often around the emperor, to serve on campaigns against external enemies or Roman rebels, and frontier-garrison troops (limitanei) which were capable of countering barbarian raids, but less and less capable of countering full-scale barbarian invasions. Constantine pursued campaigns against the tribes on the Roman frontiers—such as the Franks, the Alemanni, the Goths, and the Sarmatians—and resettled territories abandoned by his predecessors during the Crisis of the Third Century with citizens of Roman society.

Although Constantine lived much of his life as a pagan and later as a catechumen, he began to favour Christianity beginning in 312, finally becoming a Christian and being baptised by Eusebius of Nicomedia, an Arian bishop, although the Catholic Church and the Coptic Orthodox Church maintain that he was baptised by Pope Sylvester I. He played an influential role in the proclamation of the Edict of Milan in 313, which declared tolerance for Christianity in the Roman Empire. He convoked the First Council of Nicaea in 325 which produced the statement of Christian belief known as the Nicene Creed. On his orders, the Church of the Holy Sepulchre was built at the site claimed to be the tomb of Jesus in Jerusalem and was deemed the holiest place in Christendom. The papal claim to temporal power in the High Middle Ages was based on the fabricated Donation of Constantine. He has historically been referred to as the "First Christian Emperor" but while he did favour the Christian Church, some modern scholars debate his beliefs and even his comprehension of Christianity. He is venerated as a saint in Eastern Christianity, and he did much to push Christianity towards the mainstream of Roman culture.

The age of Constantine marked a distinct epoch in the history of the Roman Empire and a pivotal moment in the evolution from classical antiquity to the Middle Ages. He built a new imperial residence in the city of Byzantium, which was officially renamed New Rome, while also taking on the name Constantinople in his honour. It subsequently served as the capital of the empire for more than a thousand years—with the Eastern Roman Empire for most of that period commonly referred to retrospectively as the Byzantine Empire in English. In leaving the empire to his sons and other members of the Constantinian dynasty, Constantine's immediate political legacy was the replacement of Diocletian's Tetrarchy with the principle of dynastic succession. His memory was held in high regard during the lifetime of his children and for centuries after his reign. The medieval church held him up as a paragon of virtue, while secular rulers invoked him as a symbol of imperial legitimacy. The rediscovery of anti-Constantinian sources in the early Renaissance engendered more critical appraisals of his reign, with modern and contemporary scholarship often seeking to balance the extremes of earlier accounts.

Constantine was a ruler of major importance and has always been a controversial figure. The fluctuations in his reputation reflect the nature of the ancient sources for his reign. These are abundant and detailed, but they have been strongly influenced by the official propaganda of the period and are often one-sided. No contemporaneous histories or biographies dealing with his life and rule have survived; the nearest alternative is Eusebius's Vita Constantini, which offers a mixture of eulogy and hagiography written between 335 and 339 to extol Constantine's moral and religious virtues. The Vita creates a contentiously positive image of Constantine, and modern historians have frequently challenged its reliability. The fullest secular life of Constantine is the anonymous Origo Constantini, a work of uncertain date which focuses on military and political events to the neglect of cultural and religious matters.

Lactantius' De mortibus persecutorum, a political Christian pamphlet on the reigns of Diocletian and the Tetrarchy, provides valuable but tendentious detail on Constantine's predecessors and early life. The ecclesiastical histories of Socrates, Sozomen, and Theodoret describe the ecclesiastic disputes of Constantine's later reign. Written during the reign of Theodosius II (r. 402–450), a century after Constantine's reign, these ecclesiastical historians obscure the events and theologies of the Constantinian period through misdirection, misrepresentation, and deliberate obscurity. The contemporary writings of the orthodox Christian Athanasius of Alexandria and the ecclesiastical history of the Arian Philostorgius also survive, though their biases are no less firm.

The epitomes of Aurelius Victor (De Caesaribus), Eutropius (Breviarium), Festus (Breviarium), and the anonymous author of the Epitome de Caesaribus offer compressed secular political and military histories of the period. Although not Christian, the epitomes paint a favourable image of Constantine but omit reference to Constantine's religious policies. The Panegyrici Latini, a collection of panegyrics from the late 3rd and early 4th centuries, provides valuable information on the politics and ideology of the tetrarchic period and the early life of Constantine. In addition, contemporary architecture—such as the Arch of Constantine in Rome and palaces in Gamzigrad and Córdoba—epigraphic remains, and the coinage of the era complement the literary sources.