Ferdinand II, called Ferdinand the Catholic, was King of Aragon from 1479 until his death in 1516. As the husband and co-ruler of Queen Isabella I of Castile, he was also King of Castile from 1475 to 1504 (as Ferdinand V). He reigned jointly with Isabella over a dynastically unified Spain; together they are known as the Catholic Monarchs. Ferdinand is considered the de facto first king of Spain, and was described as such during his reign, even though, legally, Castile and Aragon remained two separate kingdoms until they were formally united by the Nueva Planta decrees issued between 1707 and 1716.

The Crown of Aragon that Ferdinand inherited in 1479 included the kingdoms of Aragon, Valencia, Majorca, Sardinia, and Sicily, as well as the Principality of Catalonia. His marriage to Isabella is regarded as the "cornerstone in the foundation of the Spanish monarchy". They played a major role in the European colonization of the Americas, sponsoring the first voyage of Christopher Columbus in 1492. That year the couple defeated Granada, the last Muslim state in Western Europe, thus completing the centuries-long Reconquista.

Following Isabella's death in 1504, the couple's daughter Joanna became queen of the Crown of Castile. That year, after a war with France, Ferdinand conquered the Kingdom of Naples. In 1507 he became regent of Castile on behalf of Joanna, who was alleged to be mentally unstable. In 1506, as part of a treaty with France, Ferdinand married Germaine of Foix, with whom he had no surviving children. In 1512 he conquered most of the Kingdom of Navarre, ruling all the territories comprising modern-day Spain until his death in 1516. He was nominally succeeded by his daughter Joanna, but power was soon assumed by her son Charles I (later Holy Roman Emperor Charles V).