| Nonnenbach | |
| Subdivision Type1: | Country |
| Subdivision Name1: | Germany |
| Subdivision Type2: | State |
| Subdivision Name2: | North Rhine-Westphalia |
| Subdivision Type3: | Reference no. |
| Subdivision Name3: | DE: 271812 |
| Length: | 6.992 km |
| Source1 Location: | Dahlem-Schmidtheim |
| Mouth Location: | Ahr near Blankenheim |
| Basin Landmarks: | Villages: Nonnenbach |
| Basin Size: | 10.885 km2 |
| Custom Label: | References |
The Nonnenbach is a seven-kilometre-long tributary of the Ahr in the area of Blankenheim in the district of Euskirchen in the German state of North Rhine-Westphalia.
The source of the Nonnenbach lies in woods about 2 kilometres east of Schmidtheim, on the far side of the B 51 and not far from gravel pits already partly filled with groundwater. It flows past the eponymous village and empties into the Ahr a good 2 kilometres south of Blankenheim. Apart from the Seidenbach and the Günzelbach it is also fed during its short course by the streams of numerous gullies, known locally as Seifen.