The Besòs (Catalan pronunciation: [bəˈzɔs]; Spanish: Besós) is a river flowing through Catalonia, Spain, formed by the confluence of the Mogent and Congost rivers.

Its full watershed includes the following cities: Aiguafreda, La Garriga, Sant Fost de Campsantelles, Canovelles, Granollers, Montmeló, Mollet del Vallès, Montcada i Reixac, Santa Coloma de Gramenet, Barcelona and Sant Adrià de Besòs.

Due to its Mediterranean climate, the river can have extreme discrepancies in flow depending on the weather.

As it passes through a highly industrialized area (Barcelona metropolitan region), it had the dubious honor of being the most contaminated river in Europe during the 1970s and 1980s.

The Fòrum Universal de les Cultures, which took place in Barcelona during 2004, allowed the creation of a recreation area called Parc Fluvial del Besòs between the cities of Barcelona, Santa Coloma de Gramenet and Sant Adrià del Besòs.