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

The Breg is the longest and biggest headstream of the river 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. Its source, which is near St. Martin's Chapel and is also called the source of the Danube or Donauquelle, is protected as a natural monument.