| Wiehl | |
| Subdivision Type1: | Country |
| Subdivision Name1: | Germany |
| Subdivision Type2: | State |
| Subdivision Name2: | North Rhine-Westphalia |
| Subdivision Type3: | Reference no. |
| Subdivision Name3: | DE: 27284 |
| Length: | 33.6km (20.9miles) |
| Source1 Location: | north of Hahn |
| Source1 Coordinates: | 50.9762°N 7.7641°W |
| Mouth Location: | near Wiehlmünden into the Agger |
| Mouth Coordinates: | 50.9875°N 7.4817°W |
| Basin Landmarks: | Large towns: Wiehl |
| Basin Size: | 140.721km2 |
| Custom Label: | References |
The Wiehl (pronounced as /de/) is a 33.6-kilometre-long, orographically left tributary of the River Agger in the German state of North Rhine-Westphalia. It is the longest river situated entirely within the county of Oberbergischer Kreis and the third longest tributary of the Agger.
The river rises in the region of Bergisches Land in the municipality of Reichshof. Its source is located about 700 metres north of the village of Hahn on the southern slopes of the Silberkuhle (514.6 m) at a height of . The Wiehl initially flows in a southerly direction and passes through the parish of Wiehl. Near Wildbergerhütte it turns towards the west. Between Nespen and Brüchermühle it is impounded by the Wiehl Dam. After passing Brüchermühle it is impounded again, this time by the Stauweiher Bieberstein. From here it flows mainly in a northwesterly direction. After passing through the parishes of Oberwiehl, Wiehl, Bielstein and Weiershagen, it empties into the Agger near Wiehlmünden at a height of .
The following is a list of Wiehl tributaries with their orographic orientation (left/right), location of their mouth and elevation at the mouth.
On 3 May 2001, following torrential rain, what were described as hundred-year floods washed away parts of some roads. Between 4 and 8 pm, 110 litres of rain fell per square metre. The average monthly precipitation in North Rhine-Westphalia is only 100 litres.