The Regen (Czech: Řezná) 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 located 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, whose meaning is unknown. The Romans called the river variously Regana (feminine gender), Reganus (masculine), and Reganum (neuter).

At Zwiesel, the Großer Regen is joined by the Kleiner Regen ("Little Regen") to form the Schwarzer Regen ("Black Regen"). The Schwarzer Regen flows through Regen and Viechtach, and is joined by the Weißer Regen ("White Regen") in Kötzting. Beyond this confluence, the river is called Regen. The river's total length, including its headstreams Großer Regen and Schwarzer Regen, is 169 km.

The Regen Valley forms the main valley crossing the Bavarian Forest; many settlements within the mountains are located along the river. Cities along the Regen river include Cham and Regensburg.