庇护九世（拉丁语：Pius IX，1792年5月13日－1878年2月7日），本名乔凡尼·玛利亚·马斯塔伊·费雷提（意大利语：Giovanni Maria Mastai-Ferretti），1846年6月16日至1878年任罗马天主教会教宗。庇护九世是天主教历史上在位时间最长的教宗，长达31年，在任期间召开了第一次梵蒂冈大公会议（1869–1870），确立教宗永无缪误的道理（Papal Infallibility，指教宗在公告信仰教理上的不能误，而非教宗永远正确），由于意大利统一过程中罗马陷落，会议被迫中止。

Pope Pius IX (Italian:Pio IX; 13 May 1792 – 7 February 1878), born Giovanni Maria Mastai-Ferretti, reigned as Pope from 16 June 1846 to his death in 1878. He was the longest-reigning elected pope in the history of the Catholic Church – over 31 years (the longest serving, un-elected pope was Peter the Apostle, posthumously considered the first pope, who was confirmed by Christ in approximately 33 AD and killed by Emperor Nero in 67 AD). During his pontificate, Pius IX convened the First Vatican Council (1869–70), which decreed papal infallibility, but the council was cut short due to the loss of the Papal States.