Pope Stephen IX (Latin: Stephanus, christened Frederick; c. 1020 – 29 March 1058) was the head of the Catholic Church and ruler of the Papal States from 3 August 1057 to his death. He was a member of the Ardenne-Verdun family, who ruled the Duchy of Lorraine, and started his ecclesiastical career as a canon in Liège. He was invited to Rome by Pope Leo IX, who made him chancellor in 1051 and one of three legates to Constantinople in 1054. The failure of their negotiations with Patriarch Michael I Cerularius of Constantinople and Archbishop Leo of Ohrid led to the permanent East-West Schism. He continued as chancellor to the next pope, Victor II, and was elected abbot of the Benedictine monastery of Montecassino.

Stephen was elected to succeed Victor on 2 August 1057. As pope, Stephen retained the Montecassino abbacy, enforced the Gregorian Reform, and continued Leo IX's efforts to expel Normans from southern Italy. He died in Florence, apparently poisoned by Romans, while endeavouring to crown his brother Godfrey the Bearded as Holy Roman emperor.