A prisoner in the Vatican (Italian: Prigioniero nel Vaticano; Latin: Captivus Vaticani) or prisoner of the Vatican described the situation of the Pope with respect to Italy during the period from the capture of Rome by the armed forces of the Kingdom of Italy on 20 September 1870 until the Lateran Treaty of 11 February 1929. Part of the process of Italian unification, the city's capture ended the millennium-old temporal rule of the popes over central Italy and allowed Rome to be designated the capital of the new nation. Although the Italians did not occupy the territories of Vatican Hill delimited by the Leonine walls and offered the creation of a city-state in the area, the Popes from Pius IX to Pius XI refused the proposal and described themselves as prisoners of the new Italian state.

As nationalism swept the Italian Peninsula in the 19th century, efforts to unify Italy were blocked in part by the Papal States, which ran through the middle of the peninsula and included the ancient capital of Rome. The Papal States were able to fend off efforts to conquer them largely through the pope's influence over the leaders of stronger European powers such as France and Austria. When Italian troops entered Rome, the Italian government reportedly intended to let the pope keep the part of Rome on the Vatican hill west of the Tiber, called Leonine City due to its walls built by Pope Leo IV, a small remaining Papal State, but Pius IX refused. One week after entering Rome, the Italian troops had taken the entire city save for the territories of Vatican Hill; the inhabitants of Rome then voted to join Italy (those living in the Vatican were allowed to vote outside of the Leonine walls).For the next 59 years, the popes refused to leave the Vatican in order to avoid any appearance of accepting the authority wielded by the Italian government over Rome as a whole. During this period, popes also refused to appear at Saint Peter's Square or at the balcony of St. Peter's Basilica facing it. Popes granted the Urbi et Orbi blessings from a balcony facing a courtyard, or from inside the basilica, and papal coronations were instead held at the Sistine Chapel. The period ended in 1929, when the Lateran Treaty created the modern state of Vatican City.

The Papal States ( PAY-pəl; Italian: Stato Pontificio), officially the State of the Church (Italian: Stato della Chiesa, Italian pronunciation: [ˈstaːto della ˈkjɛːza, ˈkjeː-]; Latin: Status Ecclesiasticus; also Dicio Pontificia), were a series of territories in the Italian Peninsula under the direct sovereign rule of the pope from 756 until 1870. They were among the major states of Italy from the 8th century until the unification of Italy, between 1859 and 1870.

The state had its origins in the rise of Christianity throughout Italy, and with it the rising influence of the Christian Church. By the mid-8th century, with the decline of the Byzantine Empire in Italy, the Papacy became effectively sovereign. Several Christian rulers – including the Frankish kings Charlemagne and Pepin the Short – further donated lands to be governed by the Church. During the Renaissance, the papal territory expanded greatly and the pope became one of Italy's most important secular rulers as well as the head of the Church. At their zenith, the Papal States covered most of the modern Italian regions of Lazio (which includes Rome), Marche, Umbria and Romagna, and portions of Emilia. These holdings were considered to be a manifestation of the temporal power of the pope, as opposed to his ecclesiastical primacy.

However, by 1861, much of the Papal States' territory had been conquered by the Kingdom of Italy. Only Lazio, including Rome, remained under the pope's temporal control. In 1870, the pope lost Lazio and Rome and had no physical territory at all, except the Basilica of St Peter and the papal residence and related buildings around the Vatican quarter of Rome, which the new Italian state did not occupy militarily, despite annexation of Lazio. In 1929 the Italian Fascist leader Benito Mussolini, the head of the Italian government, ended the "Prisoner in the Vatican" problem involving a unified Italy and the Holy See by negotiating the Lateran Treaty, signed by the two parties. This treaty recognized the sovereignty of the Holy See over a newly created international territorial entity, a city state within Rome limited to a token territory which became the Vatican City.