The Breg is a river, 46 kilometres long, in Baden-Württemberg, Germany, and the primary headstream of the Danube.

It flows through the southeast part of the Middle Black Forest and the lowlands of the Baar region.

The Breg rises at a height of 1,078 m above sea level (NN), six kilometres northwest of Furtwangen.

In its upper section, known as the Katzensteig, the Breg valley is clearly a result of glaciation, with a strikingly low gradient and landscape characterized by large Black Forest houses.

Here the Breg leaves the Black Forest and continues via Hüfingen to Donaueschingen, through wide, open countryside on the plateau of the Baar.