Merkelis Giedraitis (Polish: Melchior Giedroyć; ca. 1536 – April 6, 1609 in Alsėdžiai) was Bishop of Samogitia from 1576 to 1609. He actively combated Reformation implementing resolutions of Council of Trent in Samogitia.
Giedraitis was born into the Giedraičiai family of Lithuanian nobility. He studied at University of Königsberg (1550), Universities of Wittenberg and Tübingen (1560–1563), University of Leipzig (1563). He was ordained as a priest in 1571 and soon became a member of the diocesan chapter in Vilnius. Bishop of Samogitia Jurgis Petkūnas died in 1574. Jakub Uchański, Primate of Poland and Archbishop of Gniezno, attempted to promote his nephew to the vacancy. Walerian Protasewicz, Bishop of Vilnius, protested this nepotism and instead promoted Giedraitis, who had a clear advantage of speaking Lithuanian and Samogitian. After a two-year political struggle Giedraitis prevailed and was consecrated bishop on January 16, 1576.
Giedraitis found parishes of Diocese of Samogitia either overtaken by Protestants or neglected. He worked hard to end clerical abuses, strengthen churches and schools, and increase number of priests. He sponsored education of 12 students at Vilnius Theological Seminary and sought to establish Kražiai Theological Seminary, which was accomplished after his death. Giedraitis also invited Jesuits to Kražiai and Bernardines to Kretinga to establish the first monasteries in Samogitia. He supported use of the Lithuanian language and encouraged all priests to use it for sermons. He sponsored Mikalojus Daukša, who published a catechism (1595) and a postil (1599) in the Lithuanian language—the first Lithuanian books printed within the Grand Duchy of Lithuania. He also supported Maciej Stryjkowski, author of the first printed history of Lithuania.