Pope Pius XI (Italian: Pio XI), born Ambrogio Damiano Achille Ratti (Italian: [amˈbrɔ:dʒo daˈmja:no aˈkille ˈratti]; 31 May 1857 – 10 February 1939), was head of the Catholic Church from 6 February 1922 to his death in February 1939. He was the first sovereign of Vatican City from its creation as an independent state on 11 February 1929. He assumed as his papal motto "Pax Christi in Regno Christi", translated "The Peace of Christ in the Kingdom of Christ".

Pius XI issued numerous encyclicals, including Quadragesimo anno on the 40th anniversary of Pope Leo XIII's groundbreaking social encyclical Rerum novarum, highlighting the capitalistic greed of international finance, the dangers of socialism/communism, and social justice issues, and Quas primas, establishing the feast of Christ the King in response to anti-clericalism. The encyclical Studiorum ducem, promulgated 29 June 1923, was written on the occasion of the 6th centenary of the canonization of Thomas Aquinas, whose thought is acclaimed as central to Catholic philosophy and theology. The encyclical also singles out the Pontifical University of Saint Thomas Aquinas, Angelicum as the preeminent institution for the teaching of Aquinas: "ante omnia Pontificium Collegium Angelicum, ubi Thomam tamquam domi suae habitare dixeris" (before all others the Pontifical Angelicum College, where Thomas can be said to dwell).

To establish or maintain the position of the Catholic Church, Pius XI concluded a record number of concordats, including the Reichskonkordat with Nazi Germany, whose betrayals of which he condemned four years later in the encyclical Mit brennender Sorge ("With Burning Concern"). During his pontificate, the longstanding hostility with the Italian government over the status of the papacy and the Church in Italy was successfully resolved in the Lateran Treaty of 1929. He was unable to stop the persecution of the Church and the killing of clergy in Mexico, Spain and the Soviet Union. He canonized important saints, including Thomas More, Peter Canisius, Bernadette of Lourdes and Don Bosco. He beatified and canonized Thérèse de Lisieux, for whom he held special reverence, and gave equivalent canonization to Albertus Magnus, naming him a Doctor of the Church due to the spiritual power of his writings. He took a strong interest in fostering the participation of lay people throughout the Catholic Church, especially in the Catholic Action movement. The end of his pontificate was dominated by speaking out against Hitler and Mussolini, and defending the Catholic Church from intrusions into Catholic life and education.

Pius XI died on 10 February 1939 in the Apostolic Palace and is buried in the Papal Grotto of Saint Peter's Basilica. In the course of excavating space for his tomb, two levels of burial grounds were uncovered which revealed bones now venerated as the bones of St. Peter.