Wilhelm Kettler (20 June 1574 – 7 April 1640) 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 eastern Semigallia portion.

Born in Mitau in 1574, Wilhelm Kettler was the youngest son of Gotthard Kettler and his wife, Anna of Mecklenburg. After their father's death in 1587, Friedrich inherited the title of Duke of Courland and Semigallia, but economic control was split between Wilhelm and Friedrich. The brothers divided the duchy between themselves, and Wilhelm ruled the Courland portion, with the seat in Kuldīga. Since 1590 Wilhelm studied in University of Rostock.

In 1609, William married Princess Sophia of Brandenburg-Prussia, daughter of Albert Frederick, Duke of Prussia, receiving as a dowry the territory of Grobiņa.

In 1613 Wilhelm hosted Duke of Pomerania Philip Julius in his Kuldīga Castle. In 1615 Wilhelm ordered to kill two of his most strongest opponents brothers Magnuss and Gothard von Nolde. This caused outrage among nobility. As a result in 1617 Regional assembly (Landtag) was summoned in Skrunda Castle. Wilhelm Kettler was stripped of his title and banished from the duchy.

Wilhelm Kettler emigrated in 1617. Thereafter, his brother Friedrich became the sole ruler of the duchy.

Wilhelm traveled to Pomerania where he swore allegiance to King of Sweden Gustav II Adolph. He died in the Kucklow abbey in Pomerania on 7 April 1640. His son, Jacob Kettler, succeeded Friedrich as Duke of Courland and Semigallia in 1642.

Wilhelm's remains were returned to Courland in 1642, and he was interred in the ducal tomb on 23 February 1643.