| Altenau | |
| Mouth Location: | Alme |
| Mouth Coordinates: | 51.6714°N 8.7175°W |
| Subdivision Type1: | Country |
| Subdivision Name1: | Germany |
| Subdivision Type2: | State |
| Subdivision Name2: | North Rhine-Westphalia |
| Length: | 29.1km (18.1miles) |
| Basin Size: | 335km2 |
Altenau is a river of North Rhine-Westphalia, Germany. It is a right tributary of the Alme, which it joins in Borchen.