Saint Eustace (Latinized Eustachius or Eustathius, Greek Εὐστάθιος Πλακίδας Eustathios Plakidas) is revered as a Christian martyr. According to legend, he was martyred in AD 118, at the command of emperor Hadrian. Eustace was a pagan Roman general, who converted to Christianity after he had a vision of the cross while hunting. He lost all his wealth, was separated from his wife and sons, and went into exile in Egypt. Called back to lead the Roman army by emperor Trajan, Eustace was happily reunited with his family and restored to high social standing, but after the death of Trajan, he and his family were martyred under Hadrian for refusing to sacrifice to pagan Roman gods.

Eustace was venerated in the Byzantine Church from at least the 7th century. His veneration is attested for the Latin Church for the 8th century, but his rise to popularity in Western Europe happened in the high medieval period, during the 12th to 13th centuries. There are many versions and adaptations of his legend, in prose, in verse, and in the form of plays, in Latin, French and other languages. The saint, and scenes from his legend, were also frequently depicted in the figurative arts.

His feast day, both in Eastern and Western tradition, is on 20 September. The Armenian Apostolic Church commemorates St. Eustace on 1 October.

The original tradition of the saint's passion is Greek, the oldest surviving version (BHG 641) was composed at some point during the 5th to 7th centuries. Already in the Greek version, the later saint is a pagan general (στρατηλάτης stratēlátēs) under Trajan, called Plakidas. The Greek text also has all of the main elements found in the medieval Western accounts. Plakidas converted after he had a vision of a cross while hunting a stag and heard a divine voice prophesying his misfortune. Plakidas was baptized and took the name Eustathios ("steadfast"), his wife Tatiana took the baptismal name Theopiste. Their two sons were named Agapios and Theopistos.

Eustathios lost his estates and his wealth, and he and his family emigrated to Egypt. They could not pay for the passage, hence the ship's captain demanded to keep Theopiste for himself. Eustathios escaped with his two sons by swimming. But later, when crossing a river, his sons were taken by wild animals. Eustathios believed them dead, but they were saved.

Years later, Trajan called Eustathios back to Rome to fight the barbarians, and Eustathios once again took the rank of general and raised an army. Among the soldiers were, unbeknownst to him, his two sons. After defeating the barbarians, the general and his staff stayed in the house of an old woman. Agapios and Theopistos recognized each other. The old woman turned out to be their mother, Theopiste, who managed to escape from the ship's captain before he could dishonour her. She recognized her husband, and the entire family was reunited.

When Trajan was succeeded by Hadrian, the new emperor asked his general to sacrifice to the gods, and when Eustathios refused, he threw him and his family to the lions, but the wild animals lay down at their feet. Therefore, Hadrian ordered them to be killed in a brazen bull. The bodies of the martyrs were recovered in secrecy by Christians and buried in a "well-known place".

A Vita in French prose was composed around the middle of the 13th century.