Justin I (Latin: Iustinus; Ancient Greek: Ἰουστῖνος, romanized: Ioustînos; c. 450 – 1 August 527), also called Justin the Thracian (Latin: Iustinus Thrax; Ancient Greek: Ἰουστῖνος ὁ Θρᾷξ, romanized: Ioustînos ho Thrâix), was Roman emperor from 518 to 527. Born to a peasant family, he rose through the ranks of the army to become commander of the imperial guard and when Emperor Anastasius I Dicorus died, he out-maneouvered his rivals and was elected as his successor, in spite of being around 68 years old. His reign is significant for the founding of the Justinian dynasty that included his nephew, Justinian I, and three succeeding emperors. His consort was Empress Euphemia.

Justin was noted for his strongly Chalcedonian Christian views. This facilitated the ending of the Acacian schism between the churches of Rome and Constantinople, resulting in good relations between Justin and the papacy. Throughout his reign, he stressed the religious nature of his office and passed edicts against various Christian groups seen at the time as non-Orthodox. In foreign affairs, he used religion as an instrument of state. He endeavoured to cultivate client states on the borders of the Empire, and avoided any significant warfare until late in his reign.

Justin was born a peasant and possibly a swineherd according to legend from the hamlet Bederiana near Naissus (modern Niš, Serbia) in the region of Dardania, which was part of the Prefecture of Illyricum. He was of Thraco-Roman or Illyro-Roman descent, spoke Latin and only rudimentary Greek. His sister Vigilantia (born c. 455) married Sabbatius and had two children: the future emperor Petrus Sabbatius Justinianus (born 483) and Vigilantia (born c. 490). The younger Vigilantia married Dulcissimus (or Dulcidio) and had at least three children: the future emperor Justin II (born c. 520); the future general Marcellus; and Praejecta (born c. 520), who married the senator Areobindus.

As a young man, he and two companions left Dardania in order to escape the poverty of the region. Taking refuge in Constantinople, the capital of the Eastern Roman Empire, they possessed nothing more than the ragged clothes on their backs and a sack of bread among them. Illiterate at the time of his arrival there, Justin joined the newly formed palace guard, the excubitors. He served in various positions, campaigning against the Isaurians and the Sassanian Persians and was noticed for his bravery. Because of his ability he was successively appointed a tribune, a comes, a senator and, under the Emperor Anastasius I Dicorus, the influential position of comes excubitorum, commander of the palace guard. During this period he married Lupicina; no surviving children are recorded from this marriage. According to contemporary historian Procopius, Lupicina was a barbarian slave who had been Justin's concubine before their marriage.

During the night of 8–9 July 518, Anastasius died and his silentarii, a senior servant, summoned Justin and Celer to his deathbed. Celer was the magister officiorum (master of offices) and commander of the palace regiments of the Scholae Palatinae, a force of parade-ground display troops. By morning the event had been announced throughout the capital, Constantinople. The high officials, including John of Cappadocia, the recently appointed patriarch of Constantinople, were summoned to the Great Palace for the election of a new emperor. Meanwhile, the people gathered in the Hippodrome of Constantinople and awaited the proclamation of the name of the new emperor.

Anastasius had died childless but had a host of known relatives. This extensive family included several viable candidates for the throne. His brother Paulus had served as consul in 496. According to John Malalas, the praepositus sacri cubiculi (grand chamberlain), Amantius, had intended to have Theocritus, commander of an elite guard unit, elected to the throne. Theocritus and Amantius were relying on their control of a large military force and on buying the support of the other officials. Amantius was said to have given a substantial sum of money to Justin in order to buy his support. However, Justin controlled a smaller, but higher-quality group of soldiers, and used the money to buy support for himself. He was elected as the new emperor by the council and was proclaimed emperor in the Hippodrome.

Justin's wife became empress under the name Euphemia. The name was probably chosen for reasons of respectability. The original Euphemia was a Christian martyr during the Diocletianic Persecution. She was a local saint of Chalcedon and the Council of Chalcedon (451) had taken place in a cathedral consecrated in her name. The selection of this name was an early indication that Justin and Lupicina were fervent Chalcedonian Christians. Some of the population of the capital was supportive because of his strong Chalcedonian position on the fierce Christological debate of the time, in opposition to his predecessor's Miaphysite leanings.^{[citation needed]}

Justin cemented his position by assassinating potential opponents, especially anti-Chalcedonian supporters of Anastasius. Both Amantius and Theocritus were executed nine days after the election. Early in his reign, he also dismissed a number of officials that served emperor Anastasius, including Marinus, the praetorian prefect.