Mystici Corporis Christi (The Mystical Body of Christ) is a papal encyclical issued by Pope Pius XII on 29 June 1943 during World War II. Its main topic is the Catholic Church as the Mystical Body of Christ.

The encyclical is remembered for its statement that the Mystical Body of Christ is the Catholic Church. According to Mystici corporis, to be truly a member of the Mystical Body, one must be a member of the Catholic Church. Other Christians who erred in good faith could be unsuspectingly united to the Mystical Body by an unconscious desire and longing.

According to the Jesuit theologian Avery Dulles, Mystici Corporis was "the most comprehensive official Catholic pronouncement on the Church prior to Vatican II". Its primary writer Sebastiaan Tromp drew mainly on the first schema of Vatican I and on the encyclicals of Leo XIII. It de-emphasized papal jurisdiction, but insisted on the visibility of the church and warned against an excessively mystical understanding of the union between Christ and the Church.