The Acher is a 53.6-kilometre-long river and tributary of the Rhine in the county of Ortenau, in the south German state of Baden-Württemberg. It flows in a northwesterly direction from the Black Forest to the Rhine, between the two rivers Rench to the south and the Oos to the north.

The Black Forest is a large forested mountain range in the state of Baden-Württemberg in southwest Germany, bounded by the Rhine Valley to the west and south and close to the borders with France and Switzerland. It is the source of the Danube and Neckar rivers.