Pope Pius IX (Italian: Pio IX; born Giovanni Maria Battista Pietro Pellegrino Isidoro Mastai-Ferretti; 13 May 1792 – 7 February 1878) was head of the Catholic Church from 1846 to 1878. His reign of nearly 32 years is the longest verified of any pope in history and second only to Saint Peter according to Catholic tradition. He was notable for convoking the First Vatican Council in 1868 which defined the dogma of papal infallibility before taking a break in summer of 1870. The council never reconvened. At the same time, France started the French-Prussian War and removed the troops that protected the Papal States, which allowed the Capture of Rome by the Kingdom of Italy on 20 September 1870. Thereafter, he refused to leave Vatican City, declaring himself a "prisoner in the Vatican".

At the time of his election, he was a liberal reformer, and during his early papacy, he enacted progressive reforms, but his approach changed after the Revolutions of 1848. When his prime minister, Pellegrino Rossi, was assassinated and Pius himself was made prisoner in his own palace, he fled Rome and excommunicated all participants in the short-lived Roman Republic. After its suppression by the French army and his return in 1850, his policies and doctrinal pronouncements became increasingly conservative. He was responsible for the kidnapping of Edgardo Mortara, a six-year-old taken by force from his Jewish family who went on to become a Catholic priest in his own right and unsuccessfully attempted to convert his Jewish parents.

In his 1849 encyclical Ubi primum, he emphasized Mary's role in salvation. In 1854, he promulgated the dogma of the Immaculate Conception, articulating a long-held Catholic belief that Mary, the Mother of God, was conceived without original sin. His 1864 Syllabus of Errors was a strong condemnation of liberalism, modernism, moral relativism, secularization, separation of church and state, and other Enlightenment ideas.

His appeal for financial support revived global donations known as Peter's Pence. He strengthened the central power of the Holy See and Roman Curia over the worldwide Catholic Church, while also formalizing the pope's ultimate doctrinal authority (the dogma of papal infallibility defined in 1870). Pope John Paul II beatified him in 2000.