On 13 September 1428 at Alcolea del Cinca she married Infante Peter, Duke of Coimbra (9 December 1392 – 20 May 1449 at the Battle of Alfarrobeira), the son of King John I of Portugal and Philippa of Lancaster. He also held the titles of Regent of Portugal and the Algarve, and Sire of Montemor. They had six children. In 1438, Isabella's husband was appointed Regent of Portugal during the minority of his nephew, King Afonso V. King Afonso would later wed the eldest daughter of Isabella and Peter.