| Ferndorfbach | |
| Alternative Name: | Ferndorf--> |
| Mouth Location: | Sieg |
| Mouth Coordinates: | 50.8934°N 8.0234°W |
| Subdivision Type1: | Country |
| Length Km: | 24.4 |
| 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.