Pope Clement X (Latin: Clemens X; Italian: Clemente X; 13 July 1590 – 22 July 1676), born Emilio Bonaventura Altieri, was head of the Catholic Church and ruler of the Papal States from 29 April 1670 to his death on 22 July 1676.

Emilio Bonaventura Altieri, born in Rome in 1590, belonged to the Papal nobility. He received a doctorate in law and held various positions within the Catholic Church, including Bishop of Camerino and Superintendent of the Papal Exchequer. At the age of almost 80, he was elected Pope Clement X in 1670 after a four-month-long conclave. As Pope, he canonized and beatified various saints, promoted good relations between Christian countries, and made efforts to preserve the Altieri family name by adopting the Paoluzzi family. He also established a new tax in Rome, which led to conflicts with ambassadors and cardinals. Clement X celebrated the fourteenth jubilee of the holy year in 1675 despite his old age. During his pontificate, he created 20 cardinals, including Pietro Francesco Orsini, who later became Pope Benedict XIII.

He suspended the Portuguese Inquisition's proceedings in October 1674, challenging the techniques employed against its main victims, the New Christians.

Emilio Bonaventura Altieri was born in Rome in 1590, the son of Lorenzo Altieri and Vittoria Delfin, a noble Venetian lady. His brother was Cardinal Giambattista Altieri. The Altieri family belonged to the ancient Roman Papal nobility and had enjoyed the highest consideration at Rome for several centuries; they had occasionally contracted alliances with the Colonnas and the Orsinis. During earlier pontificates, the Altieri held many important offices and had been entrusted with several delicate missions.

Altieri received a doctorate in law from the Roman College in 1611. After finishing his studies, he was named auditor of Giovanni Battista Lancellotti in 1623, in the nuncio of Poland. He was ordained on 6 April 1624. On his return to Rome, he was named Bishop of Camerino, then governor of Loreto and of all Umbria. Pope Urban VIII (1623–44) gave him charge of the works designed to protect the territory of Ravenna from the unruly Po River.

Pope Innocent X (1644–55) sent him as nuncio to Naples, where he remained for eight years. He is credited with the re-establishment of peace after the stormy days of Masaniello. Pope Alexander VII (1655–67) confided to him a mission to Poland.

Pope Clement IX (1667–69) named him Superintendent of the Papal Exchequer (in charge of the Church's finances), and in 1667 his maestro di camera, and he was made Secretary of the Congregation of Bishops and Regulars. Just before his death, Clement IX made him a cardinal. He was then about seventy-nine years of age; and Clement IX, when making him a member of the Sacred College, said to him: "You will be our successor."

After the funeral of Pope Clement IX, sixty-two electors entered into conclave on 20 December 1669. Forty-two votes were necessary, and due to the rivalry between the French and Spanish factions, heated discussion prevailed for four months. Cardinal Giannicolò Conti was supported by twenty-two votes; Cardinal Giacomo Rospigliosi, nephew of the late pope, had thirty, or, as some say, thirty-three, with two at the accesso, so that he needed only seven more votes to gain the tiara. Cardinal Carlo Cerri obtained twenty-three votes.