Flavia Julia Helena (/ˈhɛlənə/; Ancient Greek: Ἑλένη, Helénē; c. AD 246/248 – 330), also known as Helena of Constantinople and in Christianity as Saint Helena, was a Greek Augusta of the Roman Empire and mother of Emperor Constantine the Great. She was born in the lower classes traditionally in the city of Drepanon, Bithynia, in Asia Minor, which was renamed Helenopolis.

Helena ranks as an important figure in the history of Christianity. In her final years, she made a religious tour of Syria Palaestina and Jerusalem, during which ancient tradition claims that she discovered the True Cross. The Eastern Orthodox Church, Catholic Church, Oriental Orthodox Churches, Anglican Communion, and the Lutheran Church revere her as a saint.

Though Helena's birthplace is not known with certainty, Helenopolis, then Drepanon, in Bithynia, following Procopius, is the one supported by most secondary sources, and by far the most likely candidate for her place of origin. If so, it would make her a Greek speaker or possibly bilingual. Her name is attested on coins as Flavia Helena, Flavia Julia Helena and sometimes Aelena. Joseph Vogt suggested that the name Helena was typical for the Greek-speaking part of the Roman Empire and that therefore her place of origin should be looked for in the eastern provinces of the Roman Empire. No Greek inscriptions have been attested dedicated to Helena during her lifetime, which may be because her fame was not as great in the Greek East as in the Latin West where she resided as empress. The 6th-century historian Procopius is the earliest authority for the statement that Helena was a native of Drepanon, in the province of Bithynia in Asia Minor. The name Helena appears in all areas of the Empire, but is not epigraphically attested in inscriptions of Bithynia (Helena's proposed region of origin) and it was also common in Latin-speaking areas. Procopius lived much later than the era he was describing and his description may have been actually intended as an etymological explanation about the toponym Helenopolis. On the other hand, her son Constantine renamed the city "Helenopolis" after her death around AD 330, which supports the belief that the city was indeed her birthplace. The historian Cyril Mango has argued that Helenopolis was refounded to strengthen the communication network around Constantine's new capital in Constantinople, and was renamed simply to honor Helena, not to necessarily mark her birthplace. However, according to historian Julia Hillner, Constantine's nephew, Emperor Julian, granting city status to a nearby village in Bithynia and naming it Basilinopolis in honor of his own mother, Basilina, who was undoubtedly from Bithynia, provides solid evidence that the renaming to Helenopolis marked Helena's birthplace. Constantine named two other locations after Helena: Helenopolis in Palestine, apparently due to Helena's renowned pilgrimage to the Holy Land, and the province of Helenopontus in the Pontus, which was in the same region as Drepanon, but further to the east. Two other locations have been named after Helena: a vicus Helena in northern France, and an oppidum Helena in the Pyrenees, which took its name due to Emperor Constans, a grandson of Helena, being murdered there, and corresponded with a prophecy which predicted that Constans would die in the arms of his grandmother. Other suggestions about her birthplace, without strong documentation, are Naissus (central Balkans), Caphar or Edessa (Mesopotamia), and Trier.

The bishop and historian Eusebius of Caesarea states that Helena was about 80 on her return from Palestine. Since that journey has been dated to 326–28, she was probably born around 246 to 249. Information about her social background universally suggests that she came from the lower classes. Fourth-century sources, following Eutropius' Breviarium, record that she came from a humble background. Bishop Ambrose of Milan, writing in the late 4th century was the first to call her a stabularia, a term translated as "stable-maid" or "inn-keeper". He makes this comment a virtue, calling Helena a bona stabularia, a "good stable-maid", probably to contrast her with the general suggestion of sexual laxness considered typical of that group. Other sources, especially those written after Constantine's proclamation as emperor, gloss over or ignore her background. Some ancient historians, "pagan and therefore hostile to the family ... suggested that as a girl she had been one of the supplementary amenities of her father's establishment, regularly available to his clients at a small extra charge."

Both Geoffrey of Monmouth and Henry of Huntingdon promoted a popular tradition that Helena was a British princess and the daughter of "Old King Cole" from the area of Colchester. This led to the later dedication of 135 churches in England to her, many in around the area of Yorkshire, and revived as a suggestion in the 20th century in the novel by Evelyn Waugh.

It is unknown where she first met Constantius. The historian Timothy Barnes has suggested that Constantius, while serving under Emperor Aurelian, could have met her while stationed in Asia Minor for the campaign against Zenobia. A legend has it that upon meeting they were wearing identical silver bracelets; Constantius saw her as his divinely-sent soulmate.^{[citation needed]} Barnes calls attention to an epitaph at Nicomedia of one of Aurelian's protectors, which could indicate the emperor's presence in the Bithynian region soon after AD 270. The precise legal nature of the relationship between Helena and Constantius is also unknown. The sources are equivocal on the point, sometimes calling Helena Constantius' "wife", and sometimes, following the dismissive propaganda of Constantine's rival Maxentius, calling her his "concubine". Jerome, perhaps confused by the vague terminology of his own sources, manages to do both.

Some scholars, such as the historian Jan Drijvers, assert that Constantius and Helena were joined in a common-law marriage, a cohabitation recognized in fact but not in law. Others, like Timothy Barnes, assert that Constantius and Helena were joined in an official marriage, on the grounds that the sources claiming an official marriage are more reliable.

Helena gave birth to the future emperor Constantine I on 27 February of an uncertain year soon after 270 (probably around 272). At the time, she was in Naissus (Niš, Serbia). In order to obtain a wife more consonant with his rising status, Constantius divorced Helena some time before 289, when he married Theodora, Maximian's daughter under his command. The narrative sources date the marriage to 293, when Constantius was appointed caesar (heir-apparent) of Maximian, but the Latin panegyric of 289 refers to the new couple as already married. Helena and her son were dispatched to the court of Diocletian at Nicomedia, where Constantine grew to be a member of the inner circle. Helena never remarried and lived for a time in obscurity, though close to her only son, who had a deep regard and affection for her.