The Brexbach (historically: Brachysa) is a river, just under 22 kilometres (14 mi) long, and an orographically left-hand tributary of the Saynbach in the German state of Rhineland-Palatinate.

From the confluence of its two headstreams by some fish ponds, the Brexbach initially runs along the upper edge of the municipality of Höhr-Grenzhausen.

Grenzau station is the terminus for the Brexbach Valley Railway, a line that closed passenger services in 1989 and goods trains in 1994, but which partly reopened in 2009 as a heritage railway.

Parallel to the historical railway bed runs the 16-kilometre-long (9.9 mi) Brexbach Gorge Way (Brexbachschluchtweg including the Wäller-Tour and Saynsteig paths), a very varied and, in places, challenging, hiking trail along steep and thickly wooded hillsides.

The Saynbach discharges into the Rhine in the area of Bendorf harbour.