Badalona (/ˌbædəˈloʊnə/, US also /ˌbɑːd-/, Catalan: [bəðəˈlonə], Spanish: [baðaˈlona]) is a municipality in Barcelonès county, in Catalonia (Spain). It is located to the north east of Barcelona, on the left bank of the Besòs River and on the Mediterranean Sea, in the Barcelona metropolitan area. By population, it is the third largest city in Catalonia and the twenty-third in Spain. It became a city in 1897.

The name Badalona comes from ancient Iberian word Baitolo according to the legend of several bronze coins of the end of the 2nd century BC found in the city. This word was the origin of the Latin name Baetulo that was as the Romans named the new city they founded off the coast of present Badalona. The oldest mention of the name Baetulo is from De Chorographia of Pomponius Mela (43–44 AD), who use the same name for the Besòs river (named Bissaucio during the Middle Ages). Following the Roman era, during the High Middle Ages the name Baetulo evolved to Bitulona, which was the most common name but not the only one, because it is found in the Latin written documents other versions as Bedelona, Bitilona, Betulona, Bedalona and even Vitulona. The current name is documented already in 997 and by the 14th century was the most used and generalized.

The oldest traces of settlement date from the Neolithic (about 3500–2500 BC). Archaeological finds consisting of carved stone and flint tools have been discovered in the Seriol hill and Manresà areas along with tombs with grave goods in the quarters of Sistrells and Llefià. Deposits from the Bronze and Iron Age have been found in old brickyards (dated about 1800–1500 BC) and in the masies of Can Butinyà and Can Mora in the Canyet quarter (1500–1100 BC).

Before the Roman settlement of Hispania, the territory of Badalona was populated by Iberians who lived on the Boscà hill (198 metres above the sea level), from where it is possible to see the coastline. The settlement dates to at least the 4th century BC but was totally abandoned by the 1st century AD due to Romanization. Despite the presence of the Iberians, the city's origins are usually dated to around the year 100 BC, when the Romans founded a city ex novo on Rosés hill (26,8 metres) next to the coast. The Roman town plan was based on their common scheme of the cardo and decumanus, occupying some 10 ha, with a line of walls. The Roman city was very dynamic, especially during the reign of Augustus, with a specialised viniculture that produced wine for export all over the empire. From the 3rd century, the city went into decline and was almost depopulated as people fled to rustic villae. However, the few remains from Late antiquity prove that Baetulo was never abandoned.

The current city of Badalona was formed in the 10th century when a new urban nucleus was built over the old Roman city and around the old forum and temple, which was turned into a church. At the same time, a rural nucleus emerged outside the town walls. This rural-urban dichotomy would remain until the mid-18th century.

Sant Jeroni de la Murtra Monastery, built in the 14th century, is where the Catholic Monarchs would spend their summers. According to some authors, this is also where they received Christopher Columbus after his first voyage to the Americas.

During the first half of the 19th century, Badalona remained an agricultural and fishing centre. However, this changed in 1848 with the construction of the railway connecting the cities of Mataró and Barcelona, which brought a station to the village. This furthered the industrialization and economic development of Badalona and led to the progressive decline of the traditional economic sectors. Due to the arrival of new industries, Badalona also became an important centre of the labour movement. Thanks to this, the village began to attract newcomers, doubling its population from 5,733 (1851) to 10,485 (1857). By the end of the century Badalona had around 19.000 inhabitants. The demographic growth gave rise to urban development with an ensanche plan covering all along the coast from Sant Adrià to Montgat. In 1897, the city received the title of city from the queen regent Maria Christina of Austria.

The city's demographic growth continued mostly uncontrolled in the first third of the 20th century. This entailed the construction of new neighbourhoods without urban infrastructure and poor connections to the city centre. During the dictatorship of Primo de Rivera the city council tried to alleviate these problems by developing infrastructure, building a new school, market and slaughterhouse and expanding the sewage network; however, this was not enough. During the Second Republic, Badalona was often in turmoil, as would be the case during the civil war, when the city was bombed due to its importance as an industrial centre. After the end of the war, in 1940, the mayor Frederic Xifré was executed by the Francoists.