Carlos , Prince of Asturias ( July 8 1545 -- July 24 1568 ) was the son of King Philip II of Spain by his first wife Maria Manuela of Portugal , daughter of John III of Portugal . Carlos , Prince of Asturias , also known as Don Carlos ( 8 July 1545 -- 24 July 1568 ) , was the eldest son and heir of King Philip II of Spain . His mother was Maria Manuela of Portugal , daughter of John III of Portugal .