The Aabach is a small river that runs through the Swiss cantons of Lucerne and Aargau, in the Aare catchment area.

The Aabach has its origin in Lake Baldegg, which is fed by the Ron and a number of smaller streams.

On the southern outskirts of Lenzburg a tunnel takes much of the flow of the Aabach under the town to rejoin the river in Niederlenz township.

It was built as a relief tunnel to mitigate flooding in the town, which had been occasionally severe during the spring runoff.

Beginning in the second half of the 18th century, the Aabach provided water-power that ran mills that led to the creation of industry in the valley.