The Papal Basilica of Saint Peter in the Vatican (Italian: Basilica Papale di San Pietro in Vaticano), or simply Saint Peter's Basilica (Latin: Basilica Sancti Petri), is a church of the Italian High Renaissance located in Vatican City, an independent microstate enclaved within the city of Rome. It was initially planned in the 15th century by Pope Nicholas V and then Pope Julius II to replace the aging Old St. Peter's Basilica, which was built in the fourth century by Roman emperor Constantine the Great. Construction of the present basilica began on 18 April 1506 and was completed on 18 November 1626.

Designed principally by Donato Bramante, Michelangelo, and Carlo Maderno, with piazza and fittings by Gian Lorenzo Bernini, St. Peter's is one of the most renowned works of Italian Renaissance architecture and is the largest church in the world by interior measure. While it is neither the mother church of the Catholic Church nor the cathedral of the Diocese of Rome (these equivalent titles being held by the Archbasilica of Saint John Lateran in Rome), St. Peter's is regarded as one of the holiest Catholic shrines. It has been described as "holding a unique position in the Christian world", and as "the greatest of all churches of Christendom."

Catholic tradition holds that the basilica is the burial site of Saint Peter, chief among Jesus's apostles and also the first Bishop of Rome (Pope). Saint Peter's tomb is directly below the high altar of the basilica, also known as the Altar of the Confession. For this reason, many popes, cardinals and bishops have been interred at St. Peter's since the Early Christian period.

St. Peter's is famous as a place of pilgrimage and for its liturgical functions. The pope presides at a number of liturgies throughout the year both within the basilica or the adjoining St. Peter's Square; these liturgies draw audiences numbering from 15,000 to over 80,000 people. St. Peter's has many historical associations, with the early Christian Church, the Papacy, the Protestant Reformation and Catholic CounterReformation and numerous artists, especially Michelangelo. As a work of architecture, it is regarded as the greatest building of its age. St. Peter's is one of the four churches in the world that hold the rank of Major papal basilica, all four of which are in Rome, and is also one of the Seven Pilgrim Churches of Rome. Contrary to popular misconception, it is not a cathedral because it is not the seat of a bishop; the cathedra of the pope as bishop of Rome is at the Archbasilica of Saint John Lateran.