Pope Pius XII (Italian: Pio XII; born Eugenio Maria Giuseppe Giovanni Pacelli; 2 March 1876 – 9 October 1958) was head of the Catholic Church, Primate of Italy, and sovereign of the Vatican City from 2 March 1939 until his death on 9 October 1958. He is the most recent pope to take the pontifical name "Pius".

The papacy of Pius XII was long, even by modern standards; it lasted almost 20 years, and spanned a consequential fifth of the 20th century. Pius was a diplomat pope during the destruction wrought by the Second World War, the recovery and rebuilding which followed, the beginning of the Cold War, and the early building of a new international geopolitical order, which aimed to protect human rights and maintain global peace through the establishment of international rules and institutions (such as the United Nations). Born, raised, educated, ordained, and resident for most of his life in Rome, his work in the Roman Curia—as a priest, then bishop, then cardinal—was extensive. He served as secretary of the Vatican's diplomatic Department of Extraordinary Ecclesiastical Affairs, papal nuncio to Germany, Camerlengo of the Apostolic Chamber, and Cardinal Secretary of State for the Holy See, in which capacity he worked to conclude treaties with various European and Latin American nations, including the Reichskonkordat treaty with Nazi Germany.

While the Vatican was officially neutral during the Second World War, the Reichskonkordat and Pius' leadership of the Catholic Church during the war remain the subject of controversy—including allegations of public silence and inaction concerning the fate of the Jews. Pius employed diplomacy to aid the victims of the Nazis during the war and, by directing the church to provide discreet aid to Jews and others, saved thousands of lives. Pius maintained links to the German resistance, and shared intelligence with the Allies. His strong public condemnation of genocide was considered inadequate by the Allied Powers, while the Nazis viewed him as an Allied sympathizer who had dishonoured his policy of Vatican neutrality.

During his papacy, the Catholic Church issued the Decree against Communism, declaring that Catholics who profess the atheistic and materialist doctrines of communism are to be excommunicated as apostates from the Christian faith. The church experienced severe persecution and mass deportations of Catholic clergy in the Eastern Bloc. He explicitly invoked ex cathedra papal infallibility with the dogma of the Assumption of Mary in his Apostolic constitution Munificentissimus Deus. His forty-one encyclicals include Mystici Corporis Christi, on the Church as the Mystical Body of Christ; Mediator Dei on liturgy reform; and Humani generis, in which he instructed theologians to adhere to episcopal teaching and allowed that the human body might have evolved from earlier forms. He removed, by additional international cardinal appointments, the Italian majority in the College of Cardinals in 1946.

After he died in 1958, Pope Pius XII was succeeded by John XXIII. In the process towards sainthood, his cause for canonization was opened on 18 November 1965 by Paul VI during the final session of the Second Vatican Council. He was made a Servant of God (the first threshold step towards sainthood) by John Paul II in 1990, and Benedict XVI declared Pius XII Venerable (the second step) on 19 December 2009.

Eugenio Maria Giuseppe Giovanni Pacelli was born on the second day of Lent, 2 March 1876, in Rome into an upper-class family of intense Catholic piety with a history of ties to the papacy (the "Black Nobility"). His parents were Filippo Pacelli [it] (1837–1916) and Virginia (née Graziosi) Pacelli (1844–1920). His grandfather Marcantonio Pacelli [it] had been Under-Secretary in the Papal Ministry of Finances and then Secretary of the Interior under Pope Pius IX from 1851 to 1870 and helped found the Vatican's newspaper, L'Osservatore Romano in 1861. His cousin, Ernesto Pacelli, was a key financial advisor to Pope Leo XIII; his father, Filippo Pacelli, a Franciscan tertiary, was the dean of the Roman Rota; and his brother, Francesco Pacelli, became a lay canon lawyer and the legal advisor to Pope Pius XI, in which role he negotiated the Lateran Treaty in 1929 with Benito Mussolini, bringing an end to the Roman Question.

Together with his brother Francesco (1872–1935) and his two sisters, Giuseppina (1874–1955) and Elisabetta (1880–1970), he grew up in the Parione district in the centre of Rome. Soon after the family had moved to Via Vetrina in 1880, he began school at the convent of the French Sisters of Divine Providence in the Piazza Fiammetta. The family worshipped at Chiesa Nuova. Eugenio and the other children made their First Communion at this church and Eugenio served there as an altar boy from 1886. In 1886, he also was sent to the private school of Professor Giuseppe Marchi, close to the Piazza Venezia. In 1891 Pacelli's father sent Eugenio to the Ennio Quirino Visconti Liceo Ginnasio, a state school situated in what had been the Collegio Romano, the premier Jesuit university in Rome.

In 1894, aged 18, Pacelli began his theology studies at Rome's oldest seminary, the Almo Collegio Capranica, and in November of the same year, registered to take a philosophy course at the Jesuit Pontifical Gregorian University and theology at the Pontifical Roman Athenaeum S. Apollinare. He was also enrolled at the State University, La Sapienza where he studied modern languages and history. At the end of the first academic year however, in the summer of 1895, he dropped out of both the Capranica and the Gregorian University. According to his sister Elisabetta, the food at the Capranica was to blame. Having received a special dispensation he continued his studies from home and so spent most of his seminary years as an external student. In 1899, he completed his education in Sacred Theology with a doctoral degree awarded on the basis of a short dissertation and an oral examination in Latin.