Ineffabilis Deus is an apostolic constitution by Pope Pius IX. It defines the dogma of the Immaculate Conception of the Blessed Virgin Mary. The document was promulgated on December 8, 1854, the date of the annual Solemnity of the Immaculate Conception, and followed from a positive response to the encyclical Ubi primum.

The Immaculate Conception is the belief 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 affirmed her freedom from personal sin.