The Catholic Monarchs were Queen Isabella I of Castile and King Ferdinand II of Aragon, whose marriage and joint rule marked the de facto unification of Spain. They were both from the House of Trastámara and were second cousins, being both descended from John I of Castile; to remove the obstacle that this consanguinity would otherwise have posed to their marriage under canon law, they were given a papal dispensation by Sixtus IV. They married on October 19, 1469, in the city of Valladolid; Isabella was 18 years old and Ferdinand a year younger. It is generally accepted by most scholars that the unification of Spain can essentially be traced back to the marriage of Ferdinand and Isabella. Their reign was called by W.H. Prescott "the most glorious epoch in the annals of Spain".

Isabella I, also called Isabella the Catholic, was Queen of Castile and León from 1474 until her death in 1504. She was also Queen of Aragon from 1479 until her death as the wife of King Ferdinand II. Reigning together over a dynastically unified Spain, Isabella and Ferdinand are known as the Catholic Monarchs.