The Regen (German pronunciation: [ˈʁeːɡən]; Czech: Řezná) is a river in Bavaria, Germany, and for a short distance in the Czech Republic.

The Romans called the river variously Regana (feminine gender), Reganus (masculine), and Reganum (neuter).

Pancíř at 1,060 m (3,480 ft) above sea level, in the territory of Železná Ruda in the Czech Republic.

The river crosses the Czech-German border after a few kilometres, at Bayerisch Eisenstein.

Cities along the Regen river include Cham and Regensburg.