斐迪南一世 （西班牙语：Fernando I，1017年－1065年6月24日），被称为大帝（El Magno），是纳瓦拉的桑乔三世和卡斯蒂利亚的玛攸儿的儿子，在他的舅舅死于1029年后成为卡斯蒂利亚伯爵。后来在1037年打败了他的妹夫取得了莱昂王国，他便成为了莱昂与卡斯蒂利亚的国王。1056年，他更加冕自己为西班牙皇帝。

斐迪南年仅十二岁时便登上爵位。当时的卡斯蒂利亚伯爵加西亚·桑切斯正要与莱昂国王贝尔穆多三世的妹妹桑查结婚，婚礼的地点在莱昂的圣若翰洗者教堂。当他正进入教堂的时候，却被一帮遭到放逐的卡斯蒂利亚贵族暗杀。纳瓦拉的桑乔三世作为卡斯蒂利亚的宗主，任命他的幼子斐迪南（母亲为已故加西亚的姐妹）为继任者。他更安排斐迪南与加西亚原先的未婚妻莱昂的桑查结婚。

Ferdinand I (c. 1015 – 24 December 1065), called the Great (el Magno), was the Count of Castile from his uncle's death in 1029 and the King of León after defeating his brother-in-law in 1037. According to tradition, he was the first to have himself crowned Emperor of Spain (1056), and his heirs carried on the tradition. He was a younger son of Sancho III of Navarre and Mayor of Castile, and by his father's will recognised the supremacy of his eldest brother, García Sánchez III of Navarre. While Ferdinand inaugurated the rule of the Navarrese Jiménez dynasty over western Spain, his rise to preeminence among the Christian rulers of the peninsula shifted the locus of power and culture westward after more than a century of Leonese decline. Nevertheless, "[t]he internal consolidation of the realm of León–Castilla under Fernando el Magno and [his queen] Sancha (1037–1065) is a history that remains to be researched and written."