Last master of the Livonian branch of the Teutonic Order (1559-1562) and subsequently duke of Courland (1562-1587) after the secularization of the order in Livonia.
Kettler was born in Westphalia around 1517 and grew up at the court of the archbishop of Cologne. At about the age of twenty he came to Livonia to enter the Teutonic Order, where he quickly became a member of its leadership. In the autumn of 1559 he signed a protectorate agreement with the Polish king, Sigismund II Augustus, and became master of Livonia.
Following the example of his order’s branch in Prussia, Kettler, who was already sympathetic to Lutheranism, proceeded to carry out its secularization in Livonia. On 28 November 1561 he swore loyalty to the king of Poland, and on 5 March 1562 he swore fealty as duke of Courland and Semgallia and handed over the regalia of the master. In 1562-1566 he was also confirmed as vicegerent of all the Polish possessions in Livonia. In 1566 he married Anna, duchess of Mecklenburg, and founded the dynasty of dukes of Courland.