The Rheider Au (Danish: Rejde Å or Spangebæk) is a river of Schleswig-Holstein, Germany.

The Rheider Au is a tributary of the Treene near Hollingstedt.

Its source is on the Geest near Schleswig.

In the Viking Age the route Eider - Treene - Rheider Au - Schlei served as a navigation way and/or transport or trade route between places to the north and the Baltic Sea, as commercial centres functioned (see Dorestad, Haithabu).

This article related to a river in Schleswig-Holstein is a stub.