The Arros river starts, an altitude of , in the "Forêt des Baronnies", southeast of Bagnères-de-Bigorre, in the Esparros "commune". It then flows from south to north through the Hautes-Pyrénées department. It passes through the towns of Bourg-de-Bigorre, Tournay and Chelle-Debat. It gets in the Gers department and passes through the towns of Montégut-Arros and Villecomtal-sur-Arros.