Charles X Gustav, also Carl Gustav (Swedish: Karl X Gustav), was King of Sweden from 1654 until his death. He was the son of John Casimir, Count Palatine of Zweibrücken-Kleeburg and Catherine of Sweden. After his father's death he also succeeded him as Pfalzgraf. He was married to Hedwig Eleonora of Holstein-Gottorp, who bore his son and successor, Charles XI. Charles X Gustav was the second Wittelsbach king of Sweden after the childless king Christopher of Bavaria (1441–1448) and he was the first king of the Swedish Caroline era, which had its peak during the end of the reign of his son, Charles XI. He led Sweden during the Second Northern War, enlarging the Swedish Empire. By his predecessor Christina, he was considered de facto Duke of Eyland (Öland) before ascending to the Swedish throne.
His numbering as Charles X derives from a 16th-century invention. The Swedish king Charles IX (1604–1611) chose his numeral after studying a fictitious history of Sweden. This king was the fourth actual King Charles, but has never been called Charles IV.