Isabella I also called Isabella the Catholic (Spanish: Isabel la Católica), 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.

After a struggle to claim the throne, Isabella reorganized the governmental system, brought the crime rate down, and unburdened the kingdom of the debt which her half-brother King Henry IV had left behind. Isabella's marriage to Ferdinand of Aragon in 1469 created the basis of the de facto unification of Spain. Her reforms and those she made with her husband had an influence that extended well beyond the borders of their united kingdoms.

Isabella and Ferdinand are known for being the first monarchs to be referred to as the queen and king of Spain, respectively. Their actions included completion of the Reconquista, the Alhambra Decree which ordered the mass expulsion of Jews from Spain, initiating the Spanish Inquisition, financing Christopher Columbus's 1492 voyage to the New World, and establishing the Spanish Empire, making Spain a major power in Europe and the world and ultimately ushering in the Spanish Golden Age.

Together with her husband, Isabella was granted the title of "Catholic Monarch" by Pope Alexander VI, a Spaniard. Her sainthood cause was opened in 1958, and in 1974 she was granted the title of Servant of God in the Catholic Church.