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 11.2 kilometres (7.0 mi), it flows from Pfäffikersee to the Greifensee.

Its valley is known as Aatal ("Aa Valley"), eponymous of the settlement Aathal.

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