Regnans in Excelsis is a papal bull that Pope Pius V issued on 25 February 1570. It excommunicated Queen Elizabeth I of England, referring to her as "the pretended Queen of England and the servant of crime", declared her a heretic, and released her subjects from allegiance to her, even those who had "sworn oaths to her", and excommunicated any who obeyed her orders: "We charge and command all and singular the nobles, subjects, peoples and others afore said that they do not dare obey her orders, mandates and laws. Those who shall act to the contrary we include in the like sentence of excommunication."

Pope Pius V, OP, born Antonio Ghislieri, was head of the Catholic Church and ruler of the Papal States from 7 January 1566 to his death, in May 1572. He is venerated as a saint of the Catholic Church. He is chiefly notable for his role in the Council of Trent, the Counter-Reformation, and the standardization of the Roman Rite within the Latin Church, known as Tridentine mass. Pius V declared Thomas Aquinas a Doctor of the Church.