Aa is a river of North Rhine-Westphalia, Germany. It is long and is a left tributary of the Möhne in Brilon. The river has a watershed of .
The source of Aa is in the village of Altenbüren. From here it flows in an easterly direction. At above sea level its course meanders to the north. Nearby the Hillbringse flows in from the south. Half a kilometer farther are five former mills in Aatal, called Aamühlen. The Fülsenbecke flows in near here from the north. The Aa overcomes an elevation difference of , which corresponds to an average bed slope of 9.7 percent.
The Aa is part of the larger Möhne river system, which flows through the Sauerland region and ultimately joins the Ruhr, and this valley is followed by the MöhnetalRadweg, a regional bike route that highlights the cultural and natural landscape shaped by these waterways.
See also List of rivers of North Rhine-Westphalia
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