The Portuguese Empire was the earliest and longest lived of the modern European colonial empires, spanning almost six centuries, from the capture of Ceuta in 1415 to the handover of Macau in 1999.

Portuguese explorers began exploring the coast of Africa in 1419, leveraging the latest developments in navigation, cartography and maritime technology such as the caravel, in order that they might find a sea route to the source of the lucrative spice trade. In 1488, Bartolomeu Dias rounded the Cape of Good Hope, and in 1498, Vasco da Gama reached India. In 1500, by an accidental landfall on the South American coast for some, by the crown's secret design for others, Pedro Álvares Cabral would find and lead to the establishment of the colony of Brazil. Over the following decades, Portuguese sailors continued to explore the coasts and islands of East Asia, establishing forts and trading posts as they went. By 1571, a string of outposts connected Lisbon to Nagasaki: the empire had become truly global, and in the process brought great wealth to Portugal.

Between 1580 and 1640 Portugal became the junior partner to Spain in the Iberian Union of the two countries' crowns. Though the empires continued to be administered separately, Portuguese colonies became the subject of attacks by three rival European powers hostile to Spain and envious of Iberian successes overseas: The Netherlands (which was engaged in a war of independence against Spain), England and France. With a smaller population, Portugal was unable to effectively defend its overstretched network of trading posts and factories, and so the empire began its long and gradual decline. The loss of Brazil in 1822, by then Portugal's largest and most profitable colony, at a time when independence movements were sweeping the Americas, was a blow from which Portugal and its empire would never recover.

The Scramble for Africa which began in the late 19th century left Portugal with a handful of colonies on the continent. After World War II, Portugal's right-wing dictator, António Salazar, desperately tried to keep the Portuguese Empire intact at a time when other European countries were beginning to withdraw from their colonies. In 1961 the handful of Portuguese troops garrisoned in Goa were unable to prevent Indian troops marching into the colony, but Salazar began a long and bloody war to quell anticolonialist forces in the African colonies. The unpopular war lasted until the overthrow of the Portuguese regime in 1974, known as the Carnation Revolution. The new government immediately changed policy and recognised the independence of all its colonies, including East Timor, save for Macau, which was eventually returned to China in 1999, marking the end of the Portuguese overseas empire.

The Community of Portuguese Language Countries (CPLP) is the cultural successor of the Empire.