A former kingdom of Spain, Castile comprises the two regions of Old Castile in north-western Spain, and New Castile in the centre of the country.

Previously an eastern county of the kingdom of León, Castile in the 11th century became an independent realm with its capital at Burgos and later Valladolid, and the leading force in the northern Christian states' 400-year Reconquista ("reconquest") of central and southern Spain from the Muslim rulers who had dominated the peninsula since the 8th century.

The capture of Toledo in 1085 added New Castile to the crown's territories, and the battle of Las Navas de Tolosa (1212) heralded the Muslim loss of most of the south. León was finally reunited with Castile in 1230, and the following decades saw the capture of Córdoba (1236), Murcia (1243) and Seville (1248). By the Treaty of Alcaçovas with Portugal on March 6, 1460, the ownership of the Canary Islands was transferred to Castile.

The dynastic union of Castile and Aragon in 1469, when Ferdinand II of Aragon wed Isabella of Castile, would eventually lead to the formal creation of Spain as a single entity in 1516 when their grandson Charles V assumed both thrones. See List of Spanish monarchs and Kings of Spain family tree.

The territory traditionally regarded as Castilian is now divided into the Spanish autonomous communities of Cantabria, Castile-Leon, Castile-La Mancha, Madrid and La Rioja.

The language of Castile emerged as the primary language of Spain — known to many of its speakers as castellano and in English as Castilian or Spanish. See Names given to the Spanish language.

See early history at Kingdom of León, list of Kings of Castile, and later history at History of Spain.

Two places in the United States of America named after this kingdom are: Village of Castile and Town of Castile. Both are in New York.