Sebastião I (January 20, 1554 - August 4, 1578) was a King of Portugal. He was the grandson of João III and became heir to the throne due to the death of his father, prince Joao of Portugal in 1554 (two weeks before his birth) and succeeded to the throne three years later. As Sebastian was still an infant, the regency was handled first by his Spanish grandmother, Catherine of Habsburg, and then by his uncle, Cardinal Henry of Evora. This period saw continued Portuguese colonial expansion in Angola, Mozambique, Malacca and the annexation of Macao in 1557. The young King grew up under the guidance of the Jesuits. He was a mystic who spent long periods either hunting or fasting. He was also a stubborn and reckless boy who convinced himself that he was to be captain for Jesus in a crusade against the Muslims of Africa.

Almost immediately upon attaining his majority, Sebastian began plans for a great crusade against the Moroccans of Fez. Philip II of Spain refused to be party to the plan, and postponed Sebastian's imminent marriage to a Spanish princess. The Portuguese crusaders crossed into Morocco in 1578 and, against the advice of his commanders, Sebastian marched deep inland. At Alcazarquivir (Field of the Three Kings) the Portuguese were routed by Ahmed Mohammed of Fez, and Sebastian was almost certainly killed in battle or subsequently executed. But for the Portuguese people, he just disappeared.

He then passed into legend as a great Portuguese patriot - the "sleeping King" who would return to help Portugal its darkest hour, like the British King Arthur or the German Frederick Barbarossa. In the time of Spanish occupation (1580-1640) three pretenders claimed to be King Sebastian, the last of whom - an Italian - was hanged in 1619.

Even as late as in 19th century, Sebastianist peasants in the Brazilian sertao believed that the king would return to help them against the "godless" Brazilian Republic (see Canudos for this rebellion). King Sebastian was a fragile boy, a result of marriages within the same family for many generations. For example, he only had four great-grandparents (instead of the normal 8), and four of them were descendents of King John I. There were cases of mad people in the family (his great-grandmother was Queen Joanna, the Mad). In the end, the dynasty of Avis, loved by the people, which guided Portugal to its Golden Era, killed itself by pursuing a dream: the peninsular union. The same complications from inbreeding caused the deaths of the children of John III and the madness and despair of his two grandsons (Sebastian and Charles), the last Avis-Habsburg princes.