infanta of Spain who became the instrument of her father's claims to the thrones of England and France; as archduchess of Austria, she ruled the Spanish Netherlands with her husband, Archduke Albert VII, from 1598 to 1621.

The daughter of King Philip II of Spain and his third wife, Elizabeth of Valois, Isabella was unsuccessfully proposed by her father as the Catholic heir to the throne of England after the execution (1587) of Mary, Queen of Scots, and as a claimant through her mother to the throne of France after the murder of her uncle, the French king Henry III (1589). On her betrothal to Albert, archduke of Austria, she received as dowry joint sovereignty with Albert of the Low Countries, with the stipulation that, if they had no children, the throne would revert to the Spanish crown.

Because the seven northern provinces (the United Provinces) of the Low Countries were effectively independent, Isabella and Albert, on their marriage (April 1599), had control over only the 10 southern provinces of the Netherlands (Spanish Netherlands).

Their attempts, both diplomatic and military, to reunite the 17 provinces met with failure. After Albert's death (1621), sovereignty of the Spanish Netherlands reverted to Spain, and Isabella ruled as regent of the country for her nephew, King Philip IV (Philip IV) of Spain, until her death.