| Aa | |
| Map: | Germany River Aa Map.svg |
| Mouth Location: | Werre |
| Mouth Coordinates: | 52.1236°N 8.6703°W |
| Subdivision Type1: | Country |
| Subdivision Name1: | Germany |
| Subdivision Type2: | State |
| Subdivision Name2: | North Rhine-Westphalia |
| Length: | 26km (16miles) |
| Basin Size: | 256km2 |
| Pushpin Map: | North Rhine-Westphalia |
| Pushpin Map Size: | 300 |
| Pushpin Map Caption: | North Rhine-Westphalia |
| Pushpin Map Alt: | Relief Map of North Rhine-Westphalia |
Aa (also: Westfälische Aa, "Westphalian Aa") is a river of North Rhine-Westphalia, Germany. It is a left tributary of the Werre, which it joins in Herford. It is formed by the confluence of two small streams in Bielefeld-Milse. In its upper part, it is called Johannisbach.