Joanna of Austria (Juana de Austria; 24 June 1535 – 7 September 1573) was a Princess of Portugal by marriage to John Manuel, Prince of Portugal. She served as regent of Spain to her brother Philip II of Spain during his trip to England to marry Mary I in 1554-1556, and from 1556 to 1559. She was the mother of Sebastian of Portugal. Born in the royal court of Madrid, Joanna was the daughter of Charles V, Holy Roman Emperor (who was the first king of united Spain, officially King of Aragon and King of Castile) and his consort Isabella of Portugal. Therefore, her paternal grandparents were Philip of Castile and Juana of Castile, and her maternal grandparents were Manuel I of Portugal and Maria of Aragon. She was the sister of King Philip II of Spain and Maria, Holy Roman Empress. Among others, Joanna held the titles of Archduchess of Austria, Infanta of Castile and of Aragon, and princess of Burgundy.

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