The Iberian Union refers to 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 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 after the Portuguese crisis of succession and the ensuing War of the Portuguese Succession, and lasted until the Portuguese Restoration War during which the House of Braganza was established as Portugal's new ruling dynasty.

The Habsburg king, the only element that connected the multiple kingdoms and territories, ruled by the 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 one another. Alien laws (Leyes de extranjería) determined that a national of one kingdom was a foreigner in all other kingdoms.