The founding of Barcelona is the subject of two different legends. The first attributes the founding of the city to the mythological Hercules400 years before the building of Rome. The second legend attributes the foundation of the city directly to the historical Carthaginian Hamilcar Barca, father of Hannibal, who named the city Barcino after his family in the 3rd century BC.^[24]

About 15 BC, the Romans redrew the town as a castrum (Roman military camp) centred on the "Mons Taber", a little hill near the contemporary city hall (Plaça de Sant Jaume). Under the Romans, it was a colony with the surname of Faventia,^[25] or, in full, Colonia Faventia Julia Augusta Pia Barcino^[26] or Colonia Julia Augusta Faventia Paterna Barcino. Mela^[27] mentions it among the small towns of the district, probably as it was eclipsed by its neighbour Tarraco (modern Tarragona), but it may be gathered from later writers that it gradually grew in wealth and consequence, favoured as it was with a beautiful situation and an excellent harbour.^[28] It enjoyed immunity from imperial burdens.^[29] The city minted its own coins; some from the era of Galba survive.

Some important Roman ruins are exposed under the Plaça del Rei, its entrance by the city museum (Museu d'Història de la Ciutat); the typically Roman grid plan is still visible today in the layout of the historical centre, the Barri Gòtic ("Gothic Quarter"). Some remaining fragments of the Roman walls have been incorporated into the cathedral.^[30] The cathedral, also known as the Basilica La Seu, is said to have been founded in 343. The city was conquered by the Visigoths in the early 5th century, becoming for a few years the capital of all Hispania. After being conquered by the Arabs in the early 8th century, it was reconquered in 801 by Charlemagne's son Louis, who made Barcelona the seat of the Carolingian "Spanish March" (Marca Hispanica), a buffer zoneruled by the Count of Barcelona.

The Counts of Barcelona became increasingly independent and expanded their territory to include all of Catalonia. In 1137, Aragon and the County of Barcelona merged in dynastic union^[31][32] by the marriage of Ramon Berenguer IV and Petronilla of Aragon, their titles finally borne by only one person when their son Alfonso II of Aragon ascended to the throne in 1162. His territories were later to be known as the Crown of Aragon, which conquered many overseas possessions and ruled the western Mediterranean Sea with outlying territories in Naples and Sicily and as far as Athensin the 13th century. The forging of a dynastic link between the Crowns of Aragon and Castilemarked the beginning of Barcelona's decline.

The marriage of Ferdinand II of Aragonand Isabella I of Castile in 1469 united the two royal lines. Madrid became the centre of political power whilst the colonisation of the Americas reduced the financial importance (at least in relative terms) of Mediterranean trade. Barcelona had always been the stronghold of Catalan separatism and was the center of the Catalan Revolt(1640–52) against Philip IV of Spain. The great plague of 1650–1654 halved the city's population.^[33] The Napoleonic wars left the province ravaged, but the postwar period saw the start of industrialization.

In the 18th century, a fortress was built at Montjuïc that overlooked the harbour. In 1794, this fortress was used by the French astronomer Pierre François André Méchain for observations relating to a survey stretching to Dunkirk that provided the basis of the metre.^[34] The definitive metre bar, manufactured from platinum, to the French legislative assembly on 22 June 1799.

The resistance of Barcelona to Franco's coup d'état was to have lasting effects after the defeat of the Republican government. The autonomous institutions of Cataloniawere abolished,^[35] and the use of the Catalan language in public life was suppressed. Barcelona remained the second largest city in Spain, at the heart of a region which was relatively industrialised and prosperous, despite the devastation of the civil war. The result was a large-scale immigration from poorer regions of Spain (particularly Andalucia, Murcia and Galicia), which in turn led to rapid urbanisation. Barcelona hosted the Olympic Games in 1992, which helped revitalize the city.

Barcelona is located on the northeast coast of the Iberian Peninsula, facing the Mediterranean Sea, on a plain approximately 5 km (3 mi) wide limited by the mountain range of Collserola, theLlobregat river to the southwest and the Besòs river to the north.^[37] This plain covers an area of 170 km² (66 sq mi),^[37] of which 101 km² (38.9 sq mi)^[38] are occupied by the city itself. It is 120 km (75 mi) south of the Pyrenees and the Catalan border with France.

Tibidabo, 512 m (1,680 ft) high, offers striking views over the city^[39] and is topped by the 288.4 m (946.2 ft) Torre de Collserola, a telecommunications tower that is visible from most of the city. Barcelona is peppered with small hills, most of them urbanised, that gave their name to the neighbourhoods built upon them, such as Carmel (267 m), Putxet (181 m) and Rovira (261 m). The escarpment of Montjuïc (173 m), situated to the southeast, overlooks the harbour and is topped by Montjuïc castle, a fortress built in the 17–18th centuries to control the city as a replacement for the Ciutadella. Today, the fortress is a museum and Montjuïc is home to several sporting and cultural venues, as well as Barcelona's biggest park and gardens.

The city borders on the municipalities of Santa Coloma de Gramenet and Sant Adrià de Besòs to the north; the Mediterranean Sea to the east; El Prat de Llobregat and L'Hospitalet de Llobregat to the south; and Sant Feliu de Llobregat, Sant Just Desvern, Esplugues de Llobregat, Sant Cugat del Vallès, and Montcada i Reixac to the west.

Barcelona has a Mediterranean climate^[40] (Köppen climate classification: Csa),^[41] with mild, humid winters and warm, dry summers.

Its average annual temperature is 20 °C (68 °F) during the day and 11 °C (52 °F) at night. Average annual temperature of sea is about 18 °C(64 °F). In the coldest month – January, typically the temperature ranges from 8 to 17 °C (46 to 63 °F) during the day, 2 to 10 °C (36 to 50 °F) at night and the average sea temperature is 13 °C (55 °F).^[42] In the warmest month – August, the typically temperature ranges from 25 to 31 °C (77 to 88 °F) during the day, about 20 °C (68 °F) at night and the average sea temperature is 25 °C (77 °F).^[42] Generally – "summer's" / "holiday" season lasts about six months, from May to October. Two months – April and November – are transitional, sometimes temperature exceeds 20 °C (68 °F), with average temperature of 17–18 °C (63–64 °F) during the day and 8–9 °C (46–48 °F) at night. December, January and February are the coldest months, with average temperatures around 14 °C (57 °F) during the day and 5 °C (41 °F) at night. Large fluctuations in temperature are rare, particularly in summer months. Sunshine duration is 2,524 hours per year, from 138 (average 4.5 hours of sunshine at day) in December to 310 (average 10 hours of sunshine at day) in July.