The Bidasoa (Spanish: [biðaˈsoa]; French: Bidassoa,[2] pronounced [bidasɔa]) is a river in the Basque Country of northern Spain and southern France that runs largely south to north.

Before pouring its waters into the ocean, it forms a bay called Txingudi located between these towns and Irun,[3] the site being designated Wetland of International Importance in 2002, with a total area of 1.28 km2 (0.49 sq mi).

The river comprises an area of linguistic contact, so it is pronounced differently depending on the language, namely [biˈdas̺oa] in Basque, [biðaˈsoa] in Spanish, and [bidasoa] in French.

The Navarrese tract of the river is a preferred destination for fishing enthusiasts, the river being home to several native fish species, namely eel (Anguilla anguilla), salmon (Salmo palar), trout (Salmo trutta), bullhead (Cottus gobio), Barbatula barbatula, Phoxinus phoxinus, gudgeon (Gobio lozanoi), sea lamprey (Petromyzon marinus), allis shad (Alosa alosa), flounder (Platichthys flesus) and grey mullet (Chelon labrosus), some of them declared endangered species and highly interesting (especially bullhead and salmon).

They do not make it to their goal and die before spawning, either falling prey to fishers' bait or an inability to overcome hydroelectric power stations (128 over all the course) and the 114 related dams, since 63% of them prevent migratory fish from achieving their purpose.