The Immaculate Conception is, according to Roman Catholic Dogma, the conception of the Virgin Mary without any stain ("macula" in Latin) of original sin. Under this aspect Mary is sometimes called the Immaculata (the Immaculate One), particularly in artistic contexts. The dogma says that, from the first moment of her existence, she was preserved by God from the lack of sanctifying grace that afflicts mankind, and that she was instead filled with divine grace. It is further believed that she lived a life completely free from sin. Her immaculate conception in the womb of her mother, by sexual intercourse, should not be confused with the doctrine of the virginal conception of her son Jesus.

The feast of the Immaculate Conception, celebrated on 8 December, was established as a universal feast in 1476 by Pope Sixtus IV. He did not define the doctrine as a dogma, thus leaving Roman Catholics freedom to believe in it or not without being accused of heresy; this freedom was reiterated by the Council of Trent. The existence of the feast was a strong indication of the Church's belief in the Immaculate Conception, even before its 19th century definition as a dogma.

In the Roman Catholic Church, the Solemnity of the Immaculate Conception is a Holy Day of Obligation, except where conferences of bishops have decided, with the approval of the Holy See, not to maintain it as such. It is a public holiday in some countries where Roman Catholicism is predominant.

The Immaculate Conception was solemnly defined as a dogma by Pope Pius IX in his constitution Ineffabilis Deus on 8 December 1854. The Roman Catholic Church believes the dogma is supported by Scripture (e.g., Mary's being greeted by the Angel Gabriel as "full of grace") as well as either directly or indirectly by the writings of Church Fathers such as Irenaeus of Lyons and Ambrose of Milan. Catholic theology maintains that since Jesus became incarnate of the Virgin Mary, it was fitting that she be completely free of sin for expressing her fiat.

For the Roman Catholic Church the dogma of the Immaculate Conception gained additional significance from the apparitions of Our Lady of Lourdes in 1858. At Lourdes a 14-year-old girl, Bernadette Soubirous, claimed a beautiful lady appeared to her. The lady said, "I am the Immaculate Conception", and the faithful believe her to be the Blessed Virgin Mary.

In this sense, the dogma of the Immaculate Conception defined by Pope Pius IX is also viewed as a key example of the use of sensus fidelium shared by the faithful and the Magisterium rather than pure reliance on Scripture and Tradition. The Vatican quotes in this context Fulgens Corona, where Pius XII supported such a faith:

If the popular praises of the Blessed Virgin Mary be given the careful consideration they deserve, who will dare to doubt that she, who was purer than the angels and at all times pure, was at any moment, even for the briefest instant, not free from every stain of sin?”

The Roman Catholic tradition has a well-established philosophy for the study of the Immaculate Conception and the veneration of the Blessed Virgin Mary in the field of Mariology, with Pontifical schools such as the Marianum specifically devoted to this.