| Ach | |
| Pushpin Map: | Germany |
| Pushpin Map Caption: | Mouth |
| Pushpin Map Alt: | Bavaria |
| Mouth Coordinates: | 47.7841°N 11.1077°W |
| Subdivision Type1: | Country |
| Subdivision Name1: | Germany |
| Subdivision Type2: | State |
| Subdivision Name2: | Bavaria |
| Length: | 43km (27miles) |
| Basin Size: | 120km2 |
The Ach is a tributary of the Ammer river in Bavaria, Germany. Its total length, including its source rivers the Glotzenbach and Bärenbach, is . It is the main river flowing through the Staffelsee, and flows into the Ammer east of Peißenberg.