The Bregenzer Ach (also: Bregenzer Ache) is the main river of the Bregenz Forest (Bregenzerwald) in the Austrian state of Vorarlberg. It is a tributary to Lake Constance and the River Rhine, respectively.

The source of the river is above the town of Schröcken, at an altitude of about 2,400 m (7,900 ft). The river is 67 km (42 mi) long and drains almost the entire area of the Bregenz Forest. Its mouth on Upper Lake Constance (Obersee) is near Bregenz and lies east of the outflows of the rivers Dornbirner Ach and Alpine Rhine . The outflow of Lake Constance is the High Rhine.