Gotthard von Kettler (also Ketteler, ; 1517, Wasserschloss Eggeringhausen (de), Eggeringhausen, nearby Anröchte, Kreis Soest - 17 May, 1587, Mitau) was the last Master of the Livonian Order and the first Duke of Courland and Semigallia.

Kettler was of an old Westphalian noble family and the ninth child of the German knight Gotthard Kettler zu Melrich (mentioned 1527–1556) and his wife Sophie of Nesselrode. Gotthard's older brother Wilhelm Kettler was bishop of Münster from 1553 to 1557.

In 1554 Gotthard Kettler became Komtur of Dünaburg, and in 1557 Komtur of Fellin. In 1559, during the Livonian War (1558–1582) he succeed to Wilhelm von Fürstenberg as a Master of the Teutonic Order in Livonia. When the Livonian Confederation came under increasing pressure from Tsar Ivan the Terrible, Kettler converted to Lutheranism and secularised Semigallia and Courland. On the basis of the Union of Wilno (28 November 1561), he created the Duchy of Courland and Semigallia as a vassal state of the Grand Duchy of Lithuania, which was soon merged into the Polish-Lithuanian Commonwealth.

On 11 March 1566 Kettler married Anna, Duchess of Mecklenburg (1533 – 4 July 1602), the daughter of Duke Albert VII of Mecklenburg-Güstrow and Princess Anna of Brandenburg.

Kettler died on 17 May 1587 in Mitau (Mitawa). His heirs ruled in Courland until 1737.