The Immaculate Conception is the doctrine that the Virgin Mary was free of original sin from the moment of her conception. It is one of the four Marian dogmas of the Catholic Church. Debated by medieval theologians, it was not defined as a dogma until 1854, by Pope Pius IX in the papal bull Ineffabilis Deus. While the Immaculate Conception asserts Mary's freedom from original sin, the Council of Trent, held between 1545 and 1563, had previously non-dogmatically affirmed her freedom from personal sin.

The Immaculate Conception became a popular subject in literature, but its abstract nature meant it was late in appearing as a subject in works of art. The iconography of Our Lady of the Immaculate Conception shows Mary standing, with arms outstretched or hands clasped in prayer. The feast day of the Immaculate Conception is December 8.

Many Protestant churches rejected the doctrine of the Immaculate Conception as unscriptural, though some Anglicans accept it as a pious devotion. The teaching on the Immaculate Conception among Oriental Orthodoxy varies: Shenouda III, Pope of the Coptic Orthodox Church, and the Patriarch Ignatius Zakka I of the Syriac Orthodox Church opposed the teaching, while the Eritrean Orthodox Tewahedo Church and Ethiopian Orthodox Tewahedo Church accept it.

Anne, the mother of Mary, first appears in the 2nd-century apocryphal Gospel of James. The author of the gospel borrowed from Greek tales of the childhood of heroes. For Jesus' grandmother the author drew on the more benign biblical story of Hannah—hence Anna—who conceived Samuel in her old age, thus reprising the miraculous birth of Jesus with a merely remarkable one for his mother. Anne and her husband, Joachim, are infertile, but God hears their prayers and Mary is conceived. According to Stephen J. Shoemaker, within the Gospel of James, the conception occurs without sexual intercourse between Anne and Joachim, which fits well with the Gospel of James' persistent emphasis on Mary's sacred purity, but the story does not advance the idea of an immaculate conception. The author of the Gospel of James may have based this account of Mary's conception on that of John the Baptist as recounted in the Gospel of Luke. The Eastern Orthodox Church holds that "Mary is conceived by her parents as we are all conceived".

According to church historian Frederick Holweck, writing in the Catholic Encyclopedia, Justin Martyr, Irenaeus, and Cyril of Jerusalem developed the idea of Mary as the New Eve, drawing comparison to Eve, while yet immaculate and incorrupt – that is to say, not subject to original sin. Holweck adds that Ephrem the Syrian said she was as innocent as Eve before the Fall.

Ambrose asserted Mary's incorruptibility, attributing her virginity to grace and immunity from sin. Severus, Bishop of Antioch, concurred affirming Mary's purity and immaculateness. John Damascene extended the supernatural influence of God to Mary's parents, suggesting they were purified by the Holy Spirit during her generation. According to Damascene, even the material of Mary's origin was deemed pure and holy. This perspective, which emphasized an immaculate active generation and the sanctity of the conceptio carnis, found resonance among some Western authors. Notably, the Greek Fathers did not explicitly discuss the Immaculate Conception.

By the 4th century the idea that Mary was free from sin was generally more widespread, but original sin raised the question of whether she was also free of the sin passed down from Adam. The question became acute when the feast of her conception began to be celebrated in England in the 11th century, and the opponents of the feast of Mary's conception brought forth the objection that as sexual intercourse is sinful, to celebrate Mary's conception was to celebrate a sinful event. The feast of Mary's conception originated in the Eastern Church in the 7th century, reached England in the 11th, and from there spread to Europe, where it was given official approval in 1477 and extended to the whole church in 1693; the word "immaculate" was not officially added to the name of the feast until 1854.

The doctrine of the Immaculate Conception caused a virtual civil war between Franciscans and Dominicans during the Middle Ages, with Franciscan 'Scotists' in its favour and Dominican 'Thomists' against it. The English ecclesiastic and scholar Eadmer (c. 1060 – c. 1126) reasoned that it was possible that Mary was conceived without original sin in view of God's omnipotence, and that it was also appropriate in view of her role as Mother of God: Potuit, decuit, fecit, "it was possible, it was fitting, therefore it was done". Others, including Bernard of Clairvaux (1090–1153) and Thomas Aquinas (1225–1274), objected that if Mary were free of original sin at her conception then she would have no need of redemption, making Christ's saving redemption superfluous; they were answered by Duns Scotus (1264–1308), who "developed the idea of preservative redemption as being a more perfect one: to have been preserved free from original sin was a greater grace than to be set free from sin".