| Regen | |
| Mouth Coordinates: | 49.0264°N 12.1036°W |
| Subdivision Type1: | Country |
| Length: | 190.9km (118.6miles) |
| Discharge1 Avg: | 40m3/s |
| Basin Size: | 2878km2 |
The Regen (pronounced as /de/; Czech: Řezná) is a river in Bavaria, Germany, and for a short distance in the Czech Republic. It is a left tributary of the Danube, at Regensburg, Germany.
The name in German evolved from the name in Latin, but its meaning is unknown. The Romans called the river variously (feminine gender), (masculine), and (neuter).
The source of its main headstream, the ('Great Regen'), is located in the Bohemian Forest on the southern slopes of Mt. Pancíř at 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.
At Zwiesel, the Great Regen is joined by the ('Little Regen') to form the ('Black Regen'). The Schwarzer Regen flows through Regen and Viechtach, and is joined by the ('White Regen') in Bad Kötzting. Beyond this confluence, the river is called Regen. The river's total length, including its headstreams, the Great Regen and Black Regen, is 191km (119miles). The Kleiner Regen crosses the Frauenau Reservoir.
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.