Hesse-Kassel (Hessen-Kassel) was a German principality that came into existence when the Landgraviate of Hesse was divided in 1568 upon the death of Landgrave Philip of Hesse and his eldest son Wilhelm IV inherited the northern portion and established his capital in Kassel. The Landgraviate of Hesse-Kassel expanded in 1604 when Landgrave Maurice inherited Hesse-Marburg from his uncle.

During the Thirty Years' War, Calvinist Hesse-Kassel proved Sweden's most loyal German ally. Landgrave Wilhelm V, and after his death in 1637, his widow, continued to support the French and Swedes throughout the war, and throughout maintained its own army, garrisoning many strongpoints, even while Hesse-Kassel itself was occupied by Imperial troops.

During the later 17th and 18th centuries, the Landgraves of Hesse-Kassel became famous for hiring out their army as mercenaries. Landgrave Frederick II, notably, hired out his troops to his nephew George III of Great Britain to suppress the rebellion of Britain's American colonies, the American Revolution.

The Reichsdeputationshauptschluss (Principal Decree of the Imperial Deputation) of 1803 granted Landgrave Wilhelm IX the position of an Imperial Elector (Kurfürst) and took the title Wilhelm I, Elector of Hesse. The principality thus became known as Kurhessen, although still usually referred to as Hesse-Kassel. In 1806, Wilhelm I was dispossessed by Napoleon for his support of Prussia, and Kassel became the capital of a new Kingdom of Westphalia under Napoleon's brother Jérôme. The Elector was restored as a result of Napoleon's defeat in 1813, and although the Holy Roman Empire was now defunct, Wilhelm clung to his title of Elector, hoping it would give him pre-eminence over his cousin, the Grand Duke of Hesse-Darmstadt.

Wilhelm's grandson, Elector Friedrich Wilhelm, sided with Austria in the Austro-Prussian War, and as a result, his lands were annexed by Prussia after the war, forming, along with Nassau and Frankfurt-am-Main, the new Prussian province of Hesse-Nassau.

In 1918, Prince Friedrich Karl of Hesse-Kassel, younger brother of the head of the house and a brother-in-law of Kaiser Wilhelm II, was selected by the Finnish pro-German government to be King of Finland, but he never reigned. In 1968, the head of the House of Hesse-Kassel became the Head of the entire House of Hesse due to the extinction of the Hesse-Darmstadt line.

The village of Hessen Cassel, Indiana near Fort Wayne, founded by German immigrants, is named for the Landgravcy of Hesse-Kassel.

Landgraves of Hesse-Kassel, 1568-1803

• Wilhelm IV 1568-1592
• Maurice 1592-1627 (d.1632)
• Wilhelm V 1627-1637
• Wilhelm VI 1637-1663
• Wilhelm VII 1663-1670
• Karl 1670-1730
• Friedrich I (since 1720 also King of Sweden) 1730-1751
• Wilhelm VIII 1751-1760
• Friedrich II 1760-1785
• Wilhelm IX 1785-1803 (d.1821)

Electors of Hesse(-Kassel), 1803-1866

• Wilhelm I 1803-1806

to the Kingdom of Westphalia, 1806-1813

• Wilhelm I (restored) 1813-1821
• Wilhelm II 1821-1847
• Friedrich Wilhelm 1847-1866 (d.1875)

Heads of the House of Hesse-Kassel, 1866-present

• Elector Friedrich Wilhelm 1866-1875
• Landgrave Friedrich 1875-1884
• Landgrave Alexander Friedrich 1884-1925
• Landgrave Friedrich Karl 1925-1940
• Landgrave Philipp 1940-1980
• Landgrave Moritz 1980-present