The Iberian Union was the dynastic union of the Kingdoms of Castile and Aragon and the Kingdom of Portugal under the Castilian Crown that existed between 1580 and 1640, and which brought the entire Iberian Peninsula, as well as Portuguese overseas possessions, under the Spanish Habsburg kings Philip II, Philip III and Philip IV. The union began following the Portuguese crisis of succession and the ensuing War of the Portuguese Succession, and lasted until the Portuguese Restoration War in which the House of Braganza was established as Portugal's new ruling dynasty.

The Habsburg king was the only element that connected the multiple kingdoms and territories, ruling by six separate government councils of Castile, Aragon, Portugal, Italy, Flanders, and the Indies. The governments, institutions, and legal traditions of each kingdom remained independent of each other. Alien laws (Leyes de extranjeria) determined that a national of one kingdom was a foreigner in all the other kingdoms.