The Regen (; ) is a river in Bavaria, Germany, and a left tributary of the Danube, at Regensburg, Germany. The source of its main headstream, the Großer Regen ("Big Regen"), is in the Bohemian Forest on the territory of the Czech Republic, near Železná Ruda. The river crosses the border after a few kilometres, at Bayerisch Eisenstein. The name in German evolved from the name in Latin, but its meaning is unknown. The Romans called the river variously Regana (feminine gender), Reganus (masculine), and Reganum (neuter).