The Papal Basilica of Saint Peter in the Vatican, or simply Saint Peter's Basilica, is a church of the Italian High Renaissance located in Vatican City, an independent microstate enclaved within the city of Rome, Italy. It was initially planned in the 15th century by Pope Nicholas V and then Pope Julius II to replace the ageing 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.

Pope Nicholas V, born Tommaso Parentucelli, was head of the Catholic Church and ruler of the Papal States from 6 March 1447 until his death, in March 1455. Pope Eugene IV made him a cardinal in 1446 after successful trips to Italy and Germany, and when Eugene died the next year, Parentucelli was elected in his place. He took his name Nicholas in memory of his obligations to Niccolò Albergati.