The Crown of Castile was a medieval state in the Iberian Peninsula that formed in 1230 as a result of the third and definitive union of the crowns and, some decades later, the parliaments of the kingdoms of Castile and León upon the accession of the then king, Ferdinand III of Castile, to the vacant Leonese throne. It continued to exist as a separate entity after the personal union in 1469 of the crowns of Castile and Aragon with the marriage of the Catholic Monarchs up to the dynastic union occasioned by the succession of Charles I of Spain, the Habsburg heir to both crowns in 1516.
The Indies, Islands and Mainland of the Ocean Sea were also a part of the Crown of Castile when transformed from lordships to Kingdoms of the heirs of Castile in 1506, with the treaty of Villafáfila, and upon the death of Ferdinand the Catholic.
The title of King of Castile remained in use by the Habsburg rulers during the 16th and 17th centuries. Charles I was King of Aragon, Majorca, Valencia, and Sicily; Count of Barcelona, Roussillon and Cerdagne as well as King of Castile and León, 1516–1556.