Charles X Gustav, also Carl X Gustav (Swedish: Karl X Gustav; 8 November 1622 – 13 February 1660), was King of Sweden from 1654 until his death in 1660. He was the son of John Casimir, Count Palatine (Pfalzgraf) 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. From 1655 to 1657, he also claimed the title of Grand Duke of Lithuania.
His numbering as Charles X derives from a 16th-century invention. One of his predecessors, Charles IX (1604–1611), chose his numeral after studying a fictitious history of Sweden. Charles X Gustav was the fourth actual King Charles, but has never been called Charles IV.
