The Spanish Empire, sometimes referred to as the Hispanic Monarchy or the Catholic Monarchy, was a colonial empire that existed between 1492 and 1976. In conjunction with the Portuguese Empire, it ushered in the European Age of Discovery. It achieved a global scale, controlling vast portions of the Americas, Africa, various islands in Asia and Oceania, as well as territory in other parts of Europe. It was one of the most powerful empires of the early modern period, becoming known as "the empire on which the sun never sets". At its greatest extent in the late 1700s and early 1800s, the Spanish Empire covered 13.7 million square kilometres (5.3 million square miles), making it one of the largest empires in history.

Beginning with the 1492 arrival of Christopher Columbus and continuing for over three centuries, the Spanish Empire would expand across the Caribbean Islands, half of South America, most of Central America and much of North America. In the beginning, Portugal was the only serious threat to Spanish hegemony in the New World. To end the threat of Portuguese expansion, Spain conquered Portugal and the Azores Islands from 1580 to 1582 during the War of the Portuguese Succession, resulting in the establishment of the Iberian Union, a forced union between the two crowns that lasted until 1640 when Portugal regained its independence from Spain. In 1700, Philip V became king of Spain after the death of Charles II, the last Habsburg monarch of Spain, who died without an heir.

The Magellan-Elcano circumnavigation—the first circumnavigation of the Earth—laid the foundation for Spain's Pacific empire and for Spanish control over the East Indies. The influx of gold and silver from the mines in Zacatecas and Guanajuato in Mexico and Potosí in Bolivia enriched the Spanish crown and financed military endeavors and territorial expansion. Spain was largely able to defend its territories in the Americas, with the Dutch, English, and French taking only small Caribbean islands and outposts, using them to engage in contraband trade with the Spanish populace in the Indies. Another crucial element of the empire's expansion was the financial support provided by Genoese bankers, who financed royal expeditions and military campaigns.

The Bourbon monarchy implemented reforms like the Nueva Planta decrees, which centralized power and abolished regional privileges. Economic policies promoted trade with the colonies, enhancing Spanish influence in the Americas. Socially, tensions emerged between the ruling elite and the rising bourgeoisie, as well as divisions between peninsular Spaniards and Creoles in the Americas. These factors ultimately set the stage for the independence movements that began in the early 19th century, leading to the gradual disintegration of Spanish colonial authority. By the mid-1820s, Spain had lost its territories in Mexico, Central America, and South America. By 1900, it had also lost Cuba, Puerto Rico, the Philippine Islands, and Guam in the Mariana Islands following the Spanish–American War in 1898.

With the marriage of the heirs apparent to their respective thrones Ferdinand of Aragon and Isabella of Castile created a personal union that most scholars^{[citation needed]} view as the foundation of the Spanish monarchy. The union of the Crowns of Castile and Aragon joined the economic and military power of Iberia under one dynasty, the House of Trastámara. Their dynastic alliance was important for a number of reasons, ruling jointly over a number of kingdoms and other territories, mostly in the western Mediterranean region, under their respective legal and administrative status. They successfully pursued expansion in Iberia in the Christian conquest of the Muslim Emirate of Granada, completed in 1492, for which Valencia-born Pope Alexander VI gave them the title of the Catholic Monarchs. Ferdinand of Aragon was particularly concerned with expansion in France and Italy, as well as conquests in North Africa.

With the Ottoman Turks controlling the choke points of the overland trade from Asia and the Middle East, both Spain and Portugal sought alternative routes. The Kingdom of Portugal had an advantage over the Crown of Castile, having earlier retaken territory from the Muslims. Following Portugal's earlier completion of the reconquest and its establishment of settled boundaries, it began to seek overseas expansion, first to the port of Ceuta (1415) and then by colonizing the Atlantic islands of Madeira (1418) and the Azores (1427–1452); it also began voyages down the west coast of Africa in the fifteenth century. Its rival Castile laid claim to the Canary Islands (1402) and retook territory from the Moors in 1462. The Christian rivals Castile and Portugal came to formal agreements over the division of new territories in the Treaty of Alcaçovas (1479), as well as securing the crown of Castile for Isabella whose accession was challenged militarily by Portugal.

Following the voyage of Christopher Columbus in 1492 and first major settlement in the New World in 1493, Portugal and Castile divided the world by the Treaty of Tordesillas (1494), which gave Portugal Africa and Asia, and the Western Hemisphere to Spain. The voyage of Columbus, a Genoese mariner, obtained the support of Isabella of Castile, sailing west in 1492, seeking a route to the Indies. Columbus unexpectedly encountered the New World, populated by peoples he named "Indians". Subsequent voyages and full-scale settlements of Spaniards followed, with gold beginning to flow into Castile's coffers. Managing the expanding empire became an administrative issue. The reign of Ferdinand and Isabella began the professionalization of the apparatus of government in Spain, which led to a demand for men of letters (letrados) who were university graduates (licenciados), of Salamanca, Valladolid, Complutense and Alcalá. These lawyer-bureaucrats staffed the various councils of state, eventually including the Council of the Indies and Casa de Contratación, the two highest bodies in metropolitan Spain for the government of the empire in the New World, as well as royal government in the Indies.

Portugal obtained several papal bulls that acknowledged Portuguese control over the discovered territories, but Castile also obtained from the Pope the safeguard of its rights to the Canary Islands with the bulls Romani Pontifex dated 6 November 1436 and Dominatur Dominus dated 30 April 1437. The conquest of the Canary Islands, inhabited by Guanche people, began in 1402 during the reign of Henry III of Castile, by Norman nobleman Jean de Béthencourt under a feudal agreement with the crown. The conquest was completed with the campaigns of the armies of the Crown of Castile between 1478 and 1496, when the islands of Gran Canaria (1478–1483), La Palma (1492–1493), and Tenerife (1494–1496) were subjugated. By 1504, more than 90 percent of the indigenous Canarians had been killed or enslaved.