The Aabach is a long river in Germany, a left tributary of the river Afte. It rises near the village Madfeld, part of the town Brilon, in eastern North Rhine-Westphalia near the border with Hesse. From there it flows north between mountains up to high through a forested landscape, partly in the natural park Diemelsee. After a few kilometers it flows into the artificial lake Aabachstausee. It flows to the north and empties only about further north after Bad Wünnenberg, where it flows into the Afte.