Aa is the name of a large number of small European rivers. Aa originated from an Indo-European word meaning water, and it can be seen in the German ach or aach or the North Germanic a or aa.

• Aa River (France), a river in northern France
• Aa River (Meuse), a river in North Brabant, Netherlands
• Aa of Weerijs, a river in North Brabant, Netherlands which joins the Mark at Breda
• Drentsche Aa, a river in Drenthe and Groningen, Netherlands
• Aabach (Greifensee), or Ustermer Aa, a river in Switzerland
• Aabach (Afte), formerly called the Große Aa, a river in Germany
• Aa (Möhne), a river in North Rhine-Westphalia, Germany
• Aa (Nethe), a river in North Rhine-Westphalia, Germany
• Aa (Werre), a river in North Rhine-Westphalia, Germany
• Aa, a river in Antwerp, Belgium which joins the Nete River
• Engelberger Aa, a river in Obwalden and Nidwalden, Switzerland
• Große Aa, a river in Lower Saxony, Germany
• Münstersche Aa, a river in Germany, one of several Westphalian rivers called Aa
• Sarner Aa, a river in Obwalden, Switzerland, tributary of Lake Lucerne
• Mussel Aa, a river in Groningen, Netherlands
• Pekel Aa, a river in Groningen, Netherlands
• Ruiten Aa, a river in Groningen, Netherlands
• Westerwoldsche Aa, a river in Groningen, Netherlands
• Aare, a river that runs through Bern, capital of Switzerland