Sebastian (Portuguese: Sebastião I [sɨβɐʃˈti.ɐ̃w]; 20 January 1554 – 4 August 1578) was King of Portugal from 11 June 1557 to 4 August 1578 and the penultimate Portuguese monarch of the House of Aviz.

He was the son of João Manuel, Prince of Portugal, and his wife, Joanna of Austria. He was the grandson of King John III of Portugal and Catherine of Austria, Queen of Portugal. He disappeared (presumably killed in action) in the battle of Alcácer Quibir, against the Saadi Sultanate of Morocco. Sebastian I is often referred to as the Desired (Portuguese: o Desejado) or the Hidden (o Encoberto), as the Portuguese people longed for his return to end the decline of Portugal that began after his death. He is considered to be the Portuguese example of the King asleep in mountain legend as Portuguese tradition states his return, in a foggy dawn, in Portugal's greatest hour of need.

Sebastian was born shortly after eight in the morning of 20 January 1554 (the feast of Saint Sebastian), and he was given the saint's name in commemoration. The name Sebastian was highly unusual for members of any European royal family at the time.

Sebastian was born heir apparent to the throne of Portugal, since his birth occurred two weeks after the death of his father. Soon after his birth, his mother Joanna of Spain left her infant son to serve as regent of Spain for her father, Emperor Charles V. After his abdication in 1556, she served in the same capacity for her brother Philip II of Spain. Joanna remained in Spain until her death in 1573, never to see her son again.

Sebastian succeeded to the throne at the age of three, on the death of his paternal grandfather King John III. Since he was still a child, a regency was necessary. It was handled first by his paternal grandmother, Catherine of Austria, and then by his great-uncle, Cardinal Henry of Évora. This period saw the continued Portuguese colonial expansion in Brazil, Angola, Mozambique, and Malacca, as well as the annexation of Macau in 1557.

Sebastian was a bright and lively boy. Contemporaries described him as fearless due to his great physical strength. Tall, slim, and blond, he was brought up by his grandmother Catherine. She was a domineering woman who exercised firm control over her grandson. Obedient as a child, he became obstinate and impulsive in later life.

The young king grew up under the guidance and heavy influence of the Jesuits. Aleixo de Meneses, a military man of solid reputation and former tutor and guardian of Prince John, was appointed tutor to Sebastian. Other teachers included the priest Luís Gonçalves da Câmara and his assistant, the priest Amador Rebelo.

His upbringing made Sebastian extremely devout. He carried a copy of Thomas Aquinas on a belt at his waist and was constantly accompanied by two clerics of the Theatine Order who were intent on preserving the king's innocence. As a child, Sebastian reportedly would react to visitors by running off into hiding with the monks until the visitors had gone.