The papal conclave of 1963 was convened to elect a pope, the leader of the Catholic Church, to succeed Pope John XXIII following his death on 3 June 1963.
Of the 82 members of the College of Cardinals at the time of John XXIII's death, 80 participated in the subsequent conclave. Of the 80 attending electors, 6 were cardinal bishops, 61 were cardinal priests, and 13 were cardinal deacons; 8 had been created cardinals by Pope Pius XI, 26 by Pope Pius XII, and 46 by Pope John XXIII; 21 worked in the service of the Holy See (such as in the Roman Curia), 48 were in pastoral ministry outside Rome, and 11 had retired. The oldest cardinal elector in the conclave was Francesco Morano, at the age of 90, and the youngest was Juan Landázuri Ricketts, at the age of 49. This was the last conclave before the electors were limited to cardinals under 80 years of age, a rule established by Pope Paul VI in his 1970 motu proprio Ingravescentem aetatem and his 1975 apostolic constitution Romano Pontifici eligendo.
The cardinal electors entered the Sistine Chapel to begin the conclave on 19 June 1963. On 21 June, after six ballots over three days, they elected Cardinal Giovanni Montini, the archbishop of Milan, who took the papal name Paul VI.
The data below are as of 3 June 1963, the date on which the Holy See became vacant. Cardinals belonging to institutes of consecrated life or to societies of apostolic life are indicated by the relevant post-nominal letters.
The 80 attending cardinal electors were from 28 countries. The countries with the greatest number of cardinal electors were Italy (twenty-nine), France (eight), and Spain (six).