Friedrich Kettler (Latvian: Frīdrihs Ketlers; 25 November 1569 – 17 August 1642) was Duke of Courland and Semigallia from 1587 to 1642.

Friedrich ruled the entire duchy from 1617 onward, after his brother emigrated due to conflicts with the nobility.

In 1617, the Regional Assembly of Courland (Landtag) sat in Schrunden Castle and decided that Wilhelm be stripped of his title and banished from the duchy.

The following year, Friedrich was elected as the sole Duke of Courland, and he approved a new constitution, Formula Regiminis, which gave greater rights to the aristocracy.

In 1622, the duke's residence in Mitau was surrounded by the Swedish army, forcing Friedrich to move to Goldingen (now Kuldiga).