Pope Saint Pius V (17 January 1504 – 1 May 1572), born Antonio Ghislieri (from 1518 called Michele Ghislieri, O.P.), was Pope from 1566 to 1572 and is 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 liturgy within the Latin Church. Pius V declared saint Thomas Aquinas a Doctor of the Church and patronized prominent sacred music composer Giovanni Pierluigi da Palestrina.

As Cardinal, Ghislieri gained a reputation for putting orthodoxy before personalities, prosecuting eight French Bishops for heresy. He also stood firm against nepotism, rebuking his predecessor Pope Pius IV to his face when he wanted to make a 13-year old member of his family a cardinal and subsidise a nephew from the Papal treasury.

In affairs of state, Pius V excommunicated Elizabeth I of England for schism and persecutions of English Catholics during her reign. He also arranged the formation of the Holy League, an alliance of Catholic states. Although outnumbered, the Holy League famously defeated the Ottomans, who had threatened to overrun Europe, at the Battle of Lepanto. This victory Pius V attributed to the intercession of the Blessed Virgin Mary and instituted the feast, Our Lady of Victory.