Opus Dei, formally known as The Prelature of the Holy Cross and Opus Dei, is an organization of the Roman Catholic Church that teaches the Catholic belief that everyone is called to holiness and that ordinary life is a path to sanctity. The majority of its membership are lay people, with secular priests under the governance of a prelate appointed by the Pope. Opus Dei is Latin for "Work of God", hence the organization is often referred to simply as "the Work".

Opus Dei was founded in Spain in 1928 by the Roman Catholic priest Josemaría Escrivá, and given final approval in 1950 by Pope Pius XII. In 1982, it was made into a personal prelature — its bishop's jurisdiction is not linked to one specific geographic area, but instead covers the persons in Opus Dei, wherever they are. Opus Dei is the first and so far the only Catholic organization of this type.

Opus Dei has been described as the most controversial force within the Catholic church. Allen, John, Jr. Opus Dei, 2005 and Messori, Vittorio, Opus Dei, 1997. --> Controversies about it have centered around criticisms of its recruiting methods, the alleged strict rules governing members, its acknowleged practice of mortification of the flesh, its alleged secretiveness and elitism, the alleged right-leaning politics of most of its members and the participation by many in extreme right-wing governments, especially the Francoist Government of Spain until 1978.

According to several prominent Catholic journalists, most of the criticisms against Opus Dei are mere myths created by its opponents. Several Popes and other Catholic leaders have endorsed what they see as its innovative teaching on the sanctifying value of work. In 2002, in a move interpreted by both sides of the debate as signaling his approval of Opus Dei, Pope John Paul II canonized Escrivá.

Opus Dei has about 87,000 members in more than 80 different countries. About 70% of Opus Dei members live in their private homes, leading traditional Catholic family lives with secular careers, while the other 30% are celibate, of whom the majority live in official Opus Dei centers. As well as working in more traditional charitable work through its members, Opus Dei is involved in setting up and running universities, university residences, schools, publishing houses and technical and agricultural training centres.