Zeno (/ˈziːnoʊ/; Ancient Greek: Ζήνων, romanized: Zénōn; c. 425 – 9 April 491) was Eastern Roman emperor from 474 to 475 and again from 476 to 491. His reign was plagued by domestic revolts and religious dissension, but was more successful on the foreign front. He is credited with further stabilizing the Eastern empire, while the Western Roman Empire fell following the deposition of Romulus Augustulus.

Born in Isauria, Zeno was known as Tarasis before adopting his Greek name and becoming an ally of Emperor Leo I, who saw the Isaurian general as an important counterweight against the Germanic leader Aspar. In 466, he married Leo I's daughter, Ariadne, with whom he had a son, Leo. On the death of Leo I in 474, Zeno's seven-year-old son took the throne as Leo II, with Zeno made co-emperor shortly after. Leo II died of an illness later that year, leaving Zeno as the sole emperor.

Despite his early success in making peace with the Vandals, Zeno was an unpopular emperor due to his barbarian origins. In early 475, he was forced to flee Constantinople in a revolt orchestrated in part by dowager Empress Verina, which concluded with Leo I's brother-in-law Basiliscus installing himself as emperor. Seeking refuge in his native Isauria, Zeno later took advantage of Basiliscus's own flagging popularity, convinced the generals Illus and Armatus to defect, marched on the capital and reclaimed the throne in mid-476. Meanwhile, in the western empire, Germanic leader Odoacer deposed the final Western Roman emperor, Romulus Augustulus, and sent the imperial regalia to Constantinople. He expressly acknowledged the suzerainty of Zeno over the West, making Zeno the theoretical sole ruler of a reunified empire. In return, Zeno recognised Odoacer's de facto reign in Italy.

In the late 470s and 480s Zeno faced several revolts, including one from his brother-in-law Marcianus and one from Illus, both of which he ultimately suppressed. He also attempted to deal with hostilities from the two Ostrogothic leaders, Theodoric the Amal (Theodoric the Great) and Theodoric Strabo, by playing them against each other. Following Strabo's early death, Zeno was able to achieve a lasting peace with Theodoric by sending him to Italy, where he defeated Odoacer and established the Ostrogothic Kingdom.

In religious matters, Zeno issued the Henotikon in 482 on the advice of Acacius, the Patriarch of Constantinople, in an unsuccessful attempt to reconcile the differences between the Chalcedonians and the Miaphysites over the nature of Christ. Pope Felix III condemned the document and excommunicated Acacius in 484, beginning the Acacian schism which lasted until 519. Zeno died without an heir in 491 and was succeeded by Anastasius I Dicorus, a courtier chosen by Empress Ariadne.

Zeno's original name was Tarasis or more fully Tarasikodissa (Latin: Trascalissaeus) in his native Isaurian language, meaning Tarasis son of Kodissa. Tarasis was born in Isauria, Cilicia, at Rusumblada, later renamed Zenopolis in Zeno's honour. His father was called Kodisa (as attested by his patronymic "Tarasicodissa"), his mother Lallis, his brother Longinus. Tarasis had a first wife, Arcadia, whose name indicates a relationship with the Constantinopolitan aristocracy, and whose statue was erected near the Baths of Arcadius, along the steps that led to Topoi. Near Eastern and other Christian traditions maintain that Zeno had two daughters, Hilaria and Theopiste, who followed a religious life, but historical sources attest the existence of only one son by Arcadia, called Zenon. According to ancient sources, Flavius Zeno's prestigious career—he had fought against Attila in 447 to defend Constantinople and been consul the following year—was the reason why Tarasis, another Isaurian officer, chose the Greek name Zeno when he married into the Imperial family, thus being known as Zeno when he rose to the throne. Some modern historians suggest that the Isaurian general Flavius Zeno was the father of the emperor, but there is no consensus about this, and other sources suggest that Tarasis was a member of Zeno's entourage. A Byzantine Princess, putative daughter of Zeno and first wife Arcadia, named Helen, married as his second wife Saint Vakhtang I Gorgasali, King of Iberia, and had issue.

The Isaurians were a people who lived inland from the Mediterranean coast of Anatolia, in the core of the Taurus Mountains (generally what is now the Konya/Bozkir area of Turkey). Like most borderland tribes, they were looked upon as barbarians by the Romans even though they had been Roman subjects for more than five centuries. However, being Nicene Christians rather than Arians, as the Goths and other Germanic tribes were, they were not formally barred from the throne.

According to some scholars, in the mid-460s, the Eastern Roman Emperor, Leo I, wanted to balance the weight of the Germanic component of the army, whose leader was the Alan magister militum Aspar. He thought that Tarasis and his Isaurians could be that counterweight, and called him, with many Isaurians, to Constantinople. This interpretation, however, has been contested. By the mid-460s, Arcadia and Zeno had been living at Constantinople for some time, where Lallis and Longinus also lived, the latter married to a Valeria, possibly a woman of aristocratic rank.