Wilhelm Kettler was the Duke of Courland, a Baltic German region in today's Latvia. He ruled the western Courland portion of the Duchy of Courland and Semigallia, while his brother Friedrich ruled the

Gotthard Kettler, Duke of Courland was the last Master of the Livonian Order from 1559 to 1561 and the first Duke of Courland and Semigallia from 1561 to 1587.