Casimir IV the Jagiellonian (Polish: Kazimierz Jagiellończyk) (1427 - 1492) ), of the House of Jagiello was grand duke of Lithuania from 1440 and king of Poland from 1447 until his death.

The son of king Ladislaus II and younger brother to Ladislaus III, Casimir succeeded the latter after a three-year interregnum. His marriage in 1454 to Elisabeth (daughter of Albert II of Habsburg), a member of the Habsburg house as well as the granddaughter of Sigismund, Holy Roman Emperor, eventually allowed his son Ladislaus to combine the thrones of Hungary and Bohemia. The marriage also strengthened the ties between the houses of Jagiello and Habsburg. Elizabeth became known as the mother of the Jagiellonians.

That same year, Casimir was approached by the Prussiansns for aid against the ruling Teutonic Order, which he promised; however, when he tried to annex Prussia, civil war broke out among Prussian cities, resulting in the Thirteen Years' War (1454-1466). Casimir, in alliance with the Prussian Confederation, defeated the Teutonic Order, which in the ensuing Second Treaty of Thorn recognised Polish sovereignty over western Province of Prussia and the Polish crown's overlordship over eastern Prussia. Neither was recognized by the empire, which continued as administrators of Prussia, nor by the pope. A daughter, Hedwig, was married to George the Rich Wittelsbach of Bavaria. Delegates had gone to Krakow to negotiate the marriage. Their so-called Landshut Wedding (Landshuter Hochzeit) took place in Bavaria with much pomp and celebration in 1475.

A son named Casimir was to have been married to the daughter of emperor Frederick III but he instead chose a religious life, eventually becoming canonized as St. Casimir.

Preceded by: Ladislaus III of Varna

List of Polish rulers

Succeeded by: John I Albert