Lisbon (; Portuguese: Lisboa [libo] (listen)) is the capital and the largest city of Portugal, with an estimated population of 544,851 within its administrative limits in an area of 100.05 km2. Lisbon's urban area extends beyond the city's administrative limits with a population of around 2.7 million people, being the 11th-most populous urban area in the European Union. About 3 million people live in the Lisbon metropolitan area, making it the third largest metropolitan area in the Iberian Peninsula, after Madrid and Barcelona. It represents approximately 27% of the country's population. It is mainland Europe's westernmost capital city and the only one along the Atlantic coast. Lisbon lies in the western Iberian Peninsula on the Atlantic Ocean and the River Tagus. The westernmost portions of its metro area, the Portuguese Riviera, form the westernmost point of Continental Europe, culminating at Cabo da Roca.

Lisbon is recognised as an alpha-level global city because of its importance in finance, commerce, fashion, media, entertainment, arts, international trade, education and tourism. Lisbon is one of two Portuguese cities (alongside Porto) to be recognised as a global city. Lisbon is home to three companies in the Global 2000. It is one of the major economic centres in Europe, with a growing financial sector and one of the largest container ports on Europe's Atlantic coast. Additionally, Humberto Delgado Airport served 31 million passengers in 2019, being the busiest airport in Portugal, the third busiest in the Iberian Peninsula and the 20th busiest in Europe. The motorway network and the high-speed rail system of Alfa Pendular links the main cities of Portugal to Lisbon. The city is the ninth-most-visited city in Southern Europe, after Istanbul, Rome, Barcelona, Milan, Athens, Venice, Madrid and Florence with 3,539,400 tourists in 2018. The Lisbon region has a higher GDP PPP per capita than any other region in Portugal. Its GDP amounts to US$110.3 billion and thus $32,434 per capita. The city occupies the 40th place of highest gross earnings in the world. Most of the headquarters of multinational corporations in Portugal are located in the Lisbon area. It is also the political centre of the country, as its seat of government and residence of the head of state.

Lisbon is one of the oldest cities in the world, and the second-oldest European capital city (after Athens), predating other modern European capitals by centuries. Julius Caesar made it a municipium called Felicitas Julia, adding to the name Olissipo. After the fall of the Roman Empire it was ruled by a series of Germanic tribes from the 5th century; later it was captured by the Moors in the 8th century. In 1147 Afonso Henriques conquered the city and since then it has been the political, economic and cultural centre of Portugal.

The history of Portugal can be traced from circa 400,000 years ago, when the region of present-day Portugal was inhabited by Homo heidelbergensis. Homo sapiens arrived in Portugal around 35,000 years ago.

Pre-Celtic tribes such as Lusitanians, Turduli and Oestriminis lived in the centre and north. Early in the first millennium BC, waves of Celts from Central Europe formed several ethnic groups and many tribes, dominating the northern and central area, while the south retained much of its Tartessian character, combined with the Celtici until the Roman conquest.

The Roman invasion in the 3rd century BC lasted several centuries, and developed the Roman provinces of Lusitania in the south and Gallaecia in the north. During the Dark Ages following the fall of Rome, Germanic tribes controlled the territory between the 5th and 8th centuries, including the Kingdom of the Suebi centred in Braga and the Visigothic Kingdom in the south. Eventually the Visigoths seized power in the whole of Iberia.

The 711–716 invasion by the Islamic Umayyad Caliphate conquered the Visigoth Kingdom and founded the Islamic State of Al-Andalus, gradually advancing through Iberia. At the end of the 9th century, a county based in the area of Portus Cale was established under King Alfonso III of Asturias. The Kingdom of Asturias was later divided so that northern Portugal became part of the Kingdom of León.

In 1095, Portugal broke away from the Kingdom of Galicia. At the end of the 11th century, the Burgundian knight Henry became count of Portugal and defended its independence by merging the County of Portugal and the County of Coimbra. Henry's son Afonso Henriques proclaimed himself king of Portugal in 1139. The Algarve was conquered from the Moors in 1249, and in 1255 Lisbon became the capital. Portugal's land boundaries have remained almost unchanged since then.

During the reign of King John I, the Portuguese defeated the Castilians in a war over the throne (1385) and established a political alliance with England (by the Treaty of Windsor in 1386).

From the late Middle Ages, in the 15th and 16th centuries, Portugal ascended to the status of a world power during Europe's "Age of Discovery" as it built up a vast empire. Over the following two centuries, Portugal gradually lost much of its wealth and status as other European nations took an increasing share of the spice and slave trades from the widely scattered Portuguese trading posts and territories. Signs of military decline began with the Battle of Alcácer Quibir in Morocco in 1578 and Spain's attempt to conquer England in 1588 by means of the Spanish Armada – Portugal was then in a dynastic union with Spain and had contributed ships to the Spanish fleet. Further setbacks included the destruction of much of its capital city in an earthquake in 1755, occupation during the Napoleonic Wars and the loss of its largest colony, Brazil, in 1822. From the middle of the 19th century to the late 1950s, nearly two million Portuguese left Portugal to live in Brazil and the United States.In 1910, a revolution deposed the monarchy. A military coup in 1926 installed a dictatorship that remained until another coup in 1974. The new government instituted sweeping democratic reforms and granted independence to all of Portugal's African colonies in 1975. Portugal is a founding member of the North Atlantic Treaty Organization (NATO), the Organisation for Economic Co-operation and Development (OECD), and the European Free Trade Association (EFTA). It entered the European Economic Community (now the European Union) in 1986.