Justin I (Latin: Flavius Iustinus Augustus, Ancient Greek: Ἰουστίνος; 2 February 450 – 1 August 527) was Eastern Roman Emperor from 518 to 527. He rose through the ranks of the army and ultimately became Emperor, in spite of the fact he was illiterate and almost 70 years old at the time of accession. His reign is significant for the founding of the Justinian Dynasty that included his eminent nephew Justinian I and for the enactment of laws that de-emphasized the influence of the old Roman nobility. His consort was Empress Euphemia.

Justin was a peasant and a swineherd by occupation from the region of Dardania, which is part of the Prefecture of Illyricum. He was born in a hamlet Bederiana near Scupi (modern Skopje, Macedonia). He was of Thraco-Roman or Illyro-Roman stock, spoke rudimentary Greek, and bore, like his companions and members of his family (Zimarchus, Dityvistus, Boraides, Bigleniza, Sabbatius, etc.), a Thracian name, Istok. His sister Vigilantia (b. ca 455) married Sabbatius and had two children: Petrus Sabbatius Justinianus (b. 483) and Vigilantia (b. ca 490), married to Dulcissimus and had Praejecta (b. ca 520), married to the senator Areobindus and Justin II (b. ca 520).

As a teenager, he and two companions fled from a barbaric invasion, taking refuge in Constantinople possessing nothing more than the ragged clothes on their backs and a sack of bread between them. Justin soon joined the army and, because of his ability, rose through the ranks to become a general under the Emperor Anastasius I; by the time of Anastasius' death in 518, he held the influential position of comes excubitorum, commander of the palace guard.

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