Aa is the name of a large number of small European rivers. The word is derived from the Old German aha, cognate to the Latin aqua, water (cf. German -ach). In German also Au and Aue exists, similar to å in North Germanic languages, with the meaning of moderately sized river. It is also an English noun, beloved of Scrabble players, meaning a stream.

The following are the more important streams of this name:

• in France:
  • the river Aa, in the north of France, falling into the North Sea at Gravelines, and navigable as far as Saint-Omer.
• in Switzerland:
  • Sarner Aa, a tributary of Lake Lucerne.
  • Engelberger Aa, also a tributary of Lake Lucerne.
  • Aabach, in the cantons of Lucerne and Aargau, which carries the waters of Lakes Baldegg and Hallwil into the Aar near Lenzburg.
• in Germany:
  • the Westphalian Aa (Westfälische Aa), rising in the Teutoburg Forest, and joining the Werre at Herford.
  • the Münster Aa (Münstersche Aa), a tributary of the Ems near Münster.
  • the Great Aa (Große Aa), a tributary of the Ems near Lingen.
  • and several others (Ahauser Aa, Alstätter Aa, Bocholter Aa, Grenzaa, Ibbenbürener Aa, Speller Aa, Steinfurter Aa).
• in the Netherlands:
  • Drentse Aa, a small river in the Drenthe and Groningen provinces that also flows through Groningen city.
  • Aa, a river in Noord-Brabant, flowing through Helmond and 's-Hertogenbosch.
  • Aa or Weerijs, also in Noord-Brabant, a small river near Breda.
  • several small rivers and canals in Groningen province, such as Pekel Aa, Ruiten Aa, Mussel Aa.
• in Latvia:
  • Lielupe (in German Kurländische Aa)
  • Gauja (in German Livländische Aa), both falling into the Gulf of Riga, near Riga