| Aabach | |
| Subdivision Type1: | Country |
| Subdivision Name1: | Switzerland |
| Source1: | Pfäffikersee |
| Mouth: | Greifensee |
| Mouth Coordinates: | 47.3516°N 8.6888°W |
| Progression: | Greifensee→ Glatt→ |
The Aabach (also Aa, or Ustermer Aa for disambiguation with the Mönchaltorfer Aa) is a minor river in the canton of Zürich, Switzerland. At a length of 11.2km (07miles), it flows from Pfäffikersee to the Greifensee. Its valley is known as Aatal ("Aa Valley"), eponymous of the settlement Aathal.