The R8 is a line of Rodalies de Catalunya's Barcelona commuter rail service, operated by Renfe Operadora. It runs hourly between Martorell and Granollers across the Vallès Occidental region, spanning 40 kilometres (25 mi) and eight stations. The R8 primarily uses the Castellbisbal–Mollet-Sant Fost railway, as well as the Sant Vicenç de Calders–Vilafranca del Penedès–Barcelona and Barcelona–Cerbère railways on its southern and northern ends, respectively. It is currently the only line of the Barcelona commuter rail service entirely bypassing Barcelona. The trains operating on the line are Civia electrical multiple units (EMU).

Rodalies de Catalunya is the main commuter and regional rail system in the Spanish autonomous community of Catalonia. It is administered by the Government of Catalonia and operated by the national rail operator Renfe Operadora. The system consists of 17 service lines chiefly centred in the Barcelona area, serving a total of 203 stations throughout Catalonia, with an average number of 1,000 trains running on it every day. In 2016, it had an annual ridership of 117 million.