Isabella I (Spanish: Isabel I, Old Spanish: Ysabel I; Madrigal de las Altas Torres, April 22, 1451–Medina del Campo, November 26, 1504) is Queen of Castille.

She is married to Ferdinand II of Aragon.

Their marriage becomse the basis for the political unification of Spain under their grandson, Holy Roman Emperor Charles V. After a struggle to claim her right to the throne, she reorganizes the governmental system, brings the crime rate to the lowest it has been in years, and unburdens the kingdom of the enormous debt her brother had left behind.

Her reforms and those she makes with her husband have an influence that extends well beyond the borders of their united kingdoms.

Isabella and Ferdinand are known for completing the Reconquista, ordering conversion or exile of their Muslim and Jewish subjects in the Spanish Inquisition, and for supporting and financing Christopher Columbus's 1492 voyage that leads to the opening of the New World and to the establishment of Spain as the first global power, which will dominate Europe and much of the world for more than a century.

Isabella is granted the title Servant of God by the Catholic Church in 1974.