Line 1, shortened to L1, coloured red and often simply called Línia vermella, is the second oldest Barcelona Metro line, after line L3. It is the longest line of the Barcelona Metro, and links L'Hospitalet de Llobregat and Santa Coloma de Gramenet. Originally operated by the independent Ferrocarril Metropolitano Transversal de Barcelona, it is today operated by Transports Metropolitans de Barcelona (TMB) and is part of the ATM fare-integrated main transport system. L1 is the only metro line in Spain to use Iberian gauge tracks, as used by most Spanish main line railways.

The Barcelona Metro is a network of rapid transit electrified railway lines that run mostly underground in central Barcelona and into the city's suburbs. It is part of the larger public transport system of Barcelona, the capital of Catalonia, Spain, with unified fares under the Autoritat del Transport Metropolità (ATM) scheme. As of 2014, the network is operated by two separate companies: Transports Metropolitans de Barcelona (TMB) and Ferrocarrils de la Generalitat de Catalunya (FGC). It is made up of 12 lines, combining the lines owned by the two companies. Two lines, L9 and L10, are being built at present, with both lines having different sections of each opened between 2009 and 2018. They are due to be fully completed in 2026. Three lines on the network have opened as automatic train operation/driverless vehicle systems since 2009: Line 11 being converted to driverless first, and then Lines 9 and 10, opening up driverless.