Saint Theodore (Άγιος Θεοδώρος), distinguished as Theodore of Amasea, Theodore the Recruit (Θεοδώρος ό Τήρων), and by other names, is a Christian saint and Great Martyr, particularly revered in the Eastern Orthodox Churches but also honored in Roman Catholicism and Oriental Orthodoxy. According to legend, he was a legionary in the Roman army who suffered martyrdom by immolation at Amasea in Galatian Pontus (modern Amasya, Turkey) during the Great Persecution under Diocletian in the early 4th century. Venerated by the late 4th century, he became a prominent warrior saint during the Middle Ages, attracted a great deal of additional legends including accounts of battle against dragons, and was often confused with (or was the original source of) the similar Theodore Stratelates of Heraclea.

Theodore is the English form of the Latin masculine given name Theodorus from Ancient Greek Theódōros (Θεόδωρος) from Theós (Θεός, "God") + dō̂ron (δῶρον, "gift"). In Rome, he was also known to locals as St Toto. He was eventually distinguished from other saints named Theodore as Theodore the Recruit, Theodore the Tyro, or Theodore the Soldier (Latin: Theodorus Tyro or Tiro; Ancient Greek: Θεόδωρος Τήρων or ὁ Τήρων, Theódōros Tḗrōn or ho Tḗrōn). The same name is variously anglicized as Theodore Tiron, Tiro, Tyron, Tyro, and Teron. (Nilles argued that this epithet was a later mistake and that, rather than being a recruit, Theodore's name had originally referenced his service in the Cohors Tyronum.) The saint is also distinguished as Theodore of Amasea, Theodore of Euchaita, and Theodore Martyr. The epithets are not generally needed, as Theodore Tiron is generally the intendend saint when the name "St Theodore" is used without other clarification.

Theodore was a Greek, born in Amasea. The basic legend recounts that Theodore's cohort was sent to Pontus for winter quarters. Christianity was still illegal and Galerius, prior to his 311 Edict of Toleration at Serdica, enforced his co-emperor Diocletian's Great Persecution. When the soldiers of Theodore's cohort were obliged to perform pagan sacrifice at Amasea in Galatian Pontus (modern Amasya, Turkey, about 30 miles or 48 kilometres south of the Black Coast at Sinop), he refused and recounted a confession of faith in the divinity of Jesus Christ. Rather than immediately execute him, the judges—taking pity on his youth—delayed their sentence to allow him to change his mind. Theodore then burned the city's temple of Magna Mater (Cybele), whereupon he was again arrested, tortured, and martyred by immolation. The year of his martyrdom is cited as 287 in the legenda aurea, but later tradition including Butler has the year 306. His relics were later carried to Euchaita, possibly his birthplace, by the Christian empress Eusebia sometime before her death in 360.

Iconography of a horseman with a spear overcoming evil as personified as a dragon was widespread throughout the Christian period. Iconographic representations of St Theodore as dragon-slayer are dated to as early as the 7th century, certainly by the early 10th century (the oldest certain depiction of Theodore killing a dragon is at Aghtamar, dated c. 920). Theodore is reported as having destroyed a dragon near Euchaita in a legend not younger than the late 9th century. The earliest image of St Theodore as a horseman (named in Latin) is from Vinica, North Macedonia and, if genuine, dates to the 6th or 7th century. Here, Theodore is not slaying a dragon, but holding a draco standard.

The "Christianisation" of the Thracian horseman iconography can be traced to the Cappadocian cave churches of Göreme, where frescoes of the 10th century show military saints on horseback confronting serpents with one, two or three heads. One of the earliest examples is from the church known as Mavrucan 3 (Güzelöz, Yeşilhisar [tr]), generally dated to the 10th century, which portrays two "sacred riders" confronting a two serpents twined around a tree, in a striking parallel to the Dioskuroi stela, except that the riders are now attacking the snake in the "tree of life" instead of a boar. In this example, at least, there appear to be two snakes with separate heads, but other examples of 10th-century Cappadocia show polycephalous snakes. A poorly preserved wall-painting at the Yılanlı Kilise [tr] ("Snake Church") that depicts the two saints Theodore and George attacking a dragon has been tentatively dated to the 10th century, or alternatively even to the mid-9th. A similar example, but showing three equestrian saints, Demetrius, Theodore and George, is from the "Zoodochos Pigi" chapel in central Macedonia in Greece, in the prefecture of Kilkis, near the modern village of Kolchida, dated to the 9th or 10th century.

A 12th-century depiction of Theodore as equestrian dragon-slayer is found in four muqarna panels in the nave of the Cappella Palatina in Palermo.

The dragon motif was transferred to the George legend from that of his fellow soldier saint, Saint Theodore Tiro. The transfer of the dragon iconography from Theodore, or Theodore and George as "Dioskuroi" to George on his own, first becomes tangible in the early 11th century. The oldest certain images of St. George combatting the serpent date are still found in Cappadocia, in particular the image in the church of Saint Barbara, Soganh (dated 1006 or 1021).

The emergence of Theodore Stratelates as a separate saint is attested from the late 9th century. The two Theodores were frequently depicted alongside one another in the later Byzantine period. Theodore Stratelates had a shrine at Euchaneia, but was said to have originally been from Euchaita. His "lives" are listed in Bibliotecha Hagiographica Graeca 1760–1773.