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Jacob Kettler
Portrait of Jacob Kettler, from the Stockholm National Museum
Duke of Courland and Semigallia
Reign	17 August 1642 – 1 January 1682
Predecessor	Frederick Kettler
Successor	Frederick Casimir Kettler
Born	(1610-10-28)28 October 1610 Goldingen (Kuldīga)
Died	1 January 1682(1682-01-01) (aged 71) Mitau (Jelgava)
Burial	Ducal crypt in the Jelgava Palace
Spouse	Margravine Louise Charlotte of Brandenburg ​ ​ (m. 1645; died 1676)​
Issue	Louise Elisabeth, Landgravine of Hesse-Homburg Frederick Casimir Kettler, Duke of Courland Charlotte Sophia, Abbess of Herford Maria Amalia, Landgravine of Hesse-Kassel Charles Jacob Kettler Ferdinand Kettler Alexander Kettler
House	Kettler
Father	Wilhelm Kettler
Mother	Duchess Sophie of Prussia
Religion	Lutheranism

Jacob Kettler (German: Jakob von Kettler; Latvian: Hercogs Jēkabs Ketlers; 28 October 1610 – 1 January 1682) was Duke of Courland and Semigallia from 1642 to 1682. Under his rule, Courland and Semigallia became more independent of its Polish suzerain, reached its peak in wealth, and even engaged in its own overseas colonization, making it one of the smallest, but fastest growing states in the world at that time.

Yet, in the end the results of his rule failed in the confrontation with much stronger powers both directly in the Baltic (Sweden) and overseas (Dutch Republic). A ruler "too rich and powerful to be a duke but too small and poor to be a king"^[1] could not, with his small ancestral territory and very limited resources, play the powerful role he sought in European politics of that time.

Kettler was born in Goldingen (now known as Kuldīga). He was the son of Wilhelm Kettler and Duchess Sophie of Prussia, a daughter of Albert Frederick, Duke of Prussia, and was a godson of King James I of England. While his father was exiled from the duchy, Jacob lived in the courts of Stettin and Berlin. He studied in Rostock and at the University of Leipzig and sympathized with the ideas of mercantilism.

In 1633, he led a Curonian regiment in the Smolensk War between the Polish–Lithuanian Commonwealth and Russia. In 1634, he made a grand tour of Europe, visiting Paris, London, and Amsterdam, where he studied shipbuilding. In 1638, he became the co-ruler of the duchy, and in 1642, the sole duke of Courland.

Under Kettler's rule, the duchy traded with the Netherlands, Portugal, England, France, and other nations. He started large scale reforms of the duchy's economy, improved its agriculture, opened many manufactures and started a shipbuilding industry. He founded the Fleet of Courland and Semigalla, a navy and merchant fleet. During his reign, the duchy became de facto independent of the Polish crown, because all contracts with foreign powers were signed as between independent states. After 1646, all customs administration of the duchy also was in the duke's power.

The duchy was involved in colonisation. In 1651 Jacob sent a fleet to build Fort Jacob on the Gambia River, on an island that would later be known as Kunta Kinteh Island. In 1654, he conquered Tobago with forces from the Das Wappen der Herzogin von Kurland, a double-decker ship armed with 45 cannons, carrying 25 officers, 124 Courlander soldiers, and eighty families of Latvian colonists. The colony on Tobago was named Neu Kurland ('New Courland' in German). In 1654, Duke Jacob was a party to the Treaty of Westminster.

In 1651, the duke contacted Pope Innocent X with an offer of 40 Men -of-wars and 24,000 soldiers and sailors to conquer land in Australia. Courland would receive the commercial profits of the venture, while the Pope would provide the necessary monetary backing and receive the spiritual dividends from the missionary line. The plan was dropped in 1655 with Pope Innocent X's death.^[2]

The duke was taken prisoner by the Swedes from 1658 to 1660, during the Second Northern War after the Invasion of Courland. Together with his family, he was held captive in Riga and later in Ivangorod. During this time, his colonies were attacked and lost and his fleet destroyed. After the war ended, he rebuilt the duchy's fleet and retook the island of Tobago from the Dutch. For the rest of his reign he tried to reach a pre-war level of wealth but managed to do this only partly.

The duke died in Mitau (Jelgava) on 1 January 1682.

Kettler is remembered as a fair ruler who gave opportunities to the local Latvian peasants and even knew the Latvian language.^[1]

Kettler married Princess Louise Charlotte of Brandenburg (1617–1676), the daughter of George William, Elector of Brandenburg and had issue:^[3]

Wikimedia Commons has media related to Jacob Kettler.

• Heinz Mattiesen (1974), "Jakob Kettler", Neue Deutsche Biographie (in German), vol. 10, Berlin: Duncker & Humblot, pp. 313–315; (full text online)
• Theodor Schiemann (1881), "Jakob Kettler", Allgemeine Deutsche Biographie (in German), vol. 13, Leipzig: Duncker & Humblot, pp. 540–546
• Coinage of Jacob Kettler