Isabella of Portugal (c. 1364– Marans, 4 April 1435) was the natural daughter of King Ferdinand I of Portugal, from an unknown mother. Though, it was rumored at the time that her mother was none other than the king's half-sister, Beatrice, Countess of Albuquerque,[1] with whom he was said to have had an incestuous relationship according to the chronicler Fernão Lopes.[2]