Ferdinand I, called the Great (in his time, El Magno) (1017–León, 1065), was the king of Castile from his father's death in 1035 and the king of León—through his wife—after defeating his father-in-law in 1037 until his death in 1065.

Ferdinand was the second eldest legitimate son of Sancho III of Navarre. He was barely in his teens when he was put in possession of Castile in 1028 or 1029 with his father's backing, on the murder of the last count, as the heir of his mother Munia, daughter of a previous count of Castile and sister of the deceased count. That count, Don García, was about to be married to Doña Sancha, sister of Bermudo III, king of León, but was assassinated as he was entering the church of St John the Baptist in León by a party of Castilian nobles, exiles from their own land, who had taken refuge in León.

Ferdinand now married Sancha of León instead. He reigned in Castile with the title of king from 1033, though his father, King Sancho, did not die until 1035. On 4 September 1037, when his brother-in-law Bermudo was killed in battle with him at Tamarón, Ferdinand took possession of León as well, by right of his wife who was the heiress presumptive. He overran the Moorish section of Galicia, and set up his vassal as count in what is now northern Portugal. With northern Spain consolidated, Ferdinand, in 1039, proclaimed himself emperor of Hispania. The use of the title was resented by the Emperor Henry III and Pope Victor II in 1055 as implying a claim to the headship of Christendom and as a usurpation of the Roman Empire. It did not, however, mean more than that the sovereign of León was the chief of the princes of the Iberian peninsula, and that Spain was independent of the Holy Roman Empire. Ferdinand's brothers García V of Navarre and Ramiro I of Aragón opposed his power, but were both killed in ensuing battles. After his elder brother, García's, death, Ferdinand became the "high king" of the dynasty.

Ferdinand died on the feast of Saint John the Baptist, 24 June 1065, in León, with many manifestations of ardent piety, having laid aside his crown and royal mantle, dressed in the robe of a monk and lying on a bier covered with ashes, which was placed before the altar of the church of Saint Isidore. At his death, Ferdinand divided up his kingdom between his three sons: Sancho, who received Castile; Alfonso, who received León; and Garcia, who received Galicia. His two daughters each received cities: Elvira received Toro and Urraca received Zamora. By giving them his dominion, he wanted them to abide by the split in the kingdom and respect his wishes. However, Sancho (born 1030), being the oldest, believed that he deserved more of the kingdom, and therefore sought to gain possession of the divided parts of the kingdom that had been given to his siblings.