The Magellan expedition, sometimes termed the Magellan–Elcano expedition, was a 16th-century Spanish expedition planned and led by Portuguese explorer Ferdinand Magellan. Its purpose was to secure a maritime trade route with the Moluccas, or Spice Islands, in present-day Indonesia. The expedition departed Spain in 1519 and returned there in 1522 under the command of the Spanish navigator Juan Sebastián Elcano, who completed the voyage after Magellan's death in the Philippines. One of the most significant voyages in the Age of Discovery, the nearly three-year expedition totaled 60,440 km (37,560 mi) and achieved the first circumnavigation of Earth in history. It also marked the first crossing of the Pacific by a European expedition, revealing the vast scale of that ocean, and proved that ships could sail around the world on a western sea route.

The five-ship fleet left Spain on 20 September 1519 with about 270 men. After crossing the Atlantic Ocean, the ships continued south along the eastern coast of South America. Entering the Strait of Magellan on 1 November 1520, the fleet passed through to the Pacific Ocean, which Magellan himself named Mar Pacifico. The fleet crossed the Pacific in 98 days, stopped in Guam and the Philippines, and successfully reached the Moluccas in November 1521. A much-depleted crew led by Elcano finally returned to Spain on 6 September 1522, having sailed west across the Indian Ocean, around the Cape of Good Hope through waters controlled by the Portuguese, and north along the west African coast to finally arrive in Spain.

The expedition endured many hardships, including sabotage and mutinies by the mostly Spanish crew, including Elcano himself; starvation, scurvy, storms, and hostile encounters with indigenous people also beset the voyage. Only about 40 men and one ship—the Victoria—completed the circumnavigation, and Magellan himself died in battle in the Philippines in April 1521. A series of officers succeeded him as captain-general, with Elcano eventually leading the Victoria's return trip through the Indian and Atlantic Oceans.

The expedition was funded mostly by King Charles I of Spain, with the hope that it would discover a profitable western route to the Spice Islands, as the eastern route was controlled by Portugal under the 1494 Treaty of Tordesillas. Although the expedition did find a route, it was much longer and more arduous than expected and was therefore not commercially useful. Nevertheless, the expedition is regarded as one of the greatest achievements in seamanship, and had a significant impact on the European understanding of the world.

Christopher Columbus's voyages to the West (1492–1504) had the goal of reaching the Indies and establishing direct commercial relations between Spain and the Asian kingdoms. The Spanish soon realized that the lands of the Americas were not a part of Asia, but another continent. The 1494 Treaty of Tordesillas reserved for Portugal the eastern routes that went around Africa, and Vasco da Gama and the Portuguese arrived in India in 1498.

Given the economic importance of the spice trade, Spain urgently needed to find a new commercial route to Asia. After the Junta de Toro conference of 1505, the Spanish Crown commissioned expeditions to discover a route to the west. Spanish explorer Vasco Núñez de Balboa reached the Pacific Ocean in 1513 after crossing the Isthmus of Panama, and Juan Díaz de Solís died in Río de la Plata in 1516 while exploring South America in the service of Spain.

Ferdinand Magellan was a Portuguese sailor with previous military experience in India, Malacca, and Morocco. A friend, and possible cousin, with whom Magellan sailed, Francisco Serrão, was part of the first expedition to the Moluccas, leaving from Malacca in 1511. Serrão reached the Moluccas, going on to stay on the island of Ternate and take a wife. Serrão sent letters to Magellan from Ternate, extolling the beauty and richness of the Spice Islands. These letters likely motivated Magellan to plan an expedition to the islands and would later be presented to Spanish officials when Magellan sought their sponsorship.

Historians speculate that, beginning in 1514, Magellan repeatedly petitioned King Manuel I of Portugal to fund an expedition to the Moluccas, though records are unclear. It is known that Manuel repeatedly denied Magellan's requests for a token increase to his pay, and that in late 1515 or early 1516, Manuel granted Magellan's request to be allowed to serve another master. Around this time, Magellan met the cosmographer Rui Faleiro, another Portuguese subject nursing resentment towards Manuel. The two men acted as partners in planning a voyage to the Moluccas which they would propose to the king of Spain. Magellan relocated to Seville, Spain in 1517, with Faleiro following two months later.