| Mouth location | Sieg |
|---|---|
| Mouth coordinates | 50.8934°N 8.0234°E |
| Country | Germany |
| State | North Rhine-Westphalia |
| Length | 24.4 km |
| River system | Rhine |
Ferndorfbach (vulgarly known as the Ferndorf) is a river of North Rhine-Westphalia, Germany. It flows through Kreuztal and joins the Sieg in Siegen-Weidenau. Its name relates to the village of Ferndorf, the oldest medieval part of Kreuztal town.