The Iberian Union is a historiographical term used to describe the dynastic union of the Monarchy of Spain, which in turn was itself a dynastic union of the crowns of Castile and Aragon, and the Kingdom of Portugal, and of their respective colonial empires, that existed between 1580 and 1640 and brought the entire Iberian Peninsula except Andorra, as well as Portuguese and Spanish overseas possessions, under the Spanish Habsburg monarchs 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 with the acclamation of John IV as the new King of Portugal.

Philip II, also known as Philip the Prudent, was King of Spain from 1556, King of Portugal from 1580, and King of Naples and Sicily from 1554 until his death in 1598. He was also jure uxoris King of England and Ireland from his marriage to Queen Mary I in 1554 until her death in 1558. He was also Duke of Milan from 1540. From 1555, he was Lord of the Seventeen Provinces of the Netherlands.