The Eisack (ital.: Isarco) river is the second largest river in the South Tyrol, the mainly German-speaking region of Italy. It source is near the Brenner at an altitude of about 1990 m above sea level. After about 96 km, it meets the Etsch-Adige river south of Bozen-Bolzano. The Eisack draws water from an area of about 4.200 km².

The more important towns and villages along the course of the river are Sterzing-Vipiteno, Franzensfeste-Fortezza , Brixen-Bressanone, Klausen-Chiusa and Waidbruck-Ponte Gardena . Several smaller creeks flow into the Eisack, including the Ridnauner Bach , the Pflerscher Bach , the Pfitscher Bach , Rienz , the Villnösser Bach , the Grödner Bach , the Braibach (also known as Tierser Bach), the Eggentaler Bach and the Talfer flowing from the Sarntal .

The Eisack is used extensively for the production of electricity and is dammed near Franzensfeste, Klausen and Waidbruck.