The Breitach is a 24-kilometre-long (15 mi) mountain river, the southwestern (left) source of the Iller in the Allgäu Alps , in the states of Vorarlberg (Austria) and Bavaria (Germany).

The river originates in Baad, a part of Mittelberg , in the Kleinwalsertal as the union of three smaller source streams. It flows in the northwestern direction through the valley that in earlier times was called Breitachtal ("Breitach Valley") after the river. At the Walserschanze, the Austrian-German border, the Breitach reaches German territory and curves through the narrow Breitachklamm. Then, the Starzlach flows from the west into the Breitach. At the so-called Illerursprung ("Iller origin") in Oberstdorf , the Breitach, the Stillach and the Trettach flow together forming the Iller.