Austrian statesman, bishop of Vienna and later a cardinal, who tried to promote religious toleration during the Counter-Reformation in Austria. Converted from Protestantism by the Jesuits, he became an outstanding preacher and served as bishop of Vienna from the 1590s.

Klesl became the most trusted adviser of the Habsburg archduke Matthias, king of Hungary and of Bohemia, and helped to procure his patron's election as Holy Roman emperor (June 13, 1612). He was then appointed director of the privy council and was allowed by Matthias to conduct most of the secular political affairs of the empire. Klesl was made a cardinal secretly in 1615 and publicly the next year. He hoped to reconcile the religious factions within the empire by means of reciprocal concessions. His conciliatory attitude was resented by the German Catholic princes and by the archdukes Maximilian and Ferdinand (afterward emperor as Ferdinand II). When Klesl recommended concessions to the Bohemian Protestants, he was seized by the archdukes in 1618 and imprisoned. In 1627 Ferdinand II allowed Klesl to return as bishop to Vienna and restored most of his fortune.