The Eider (lat. Egdor or Egdore) is the longest river of Schleswig-Holstein, Germany. The river starts south of Kiel near the shores of the Baltic Sea, but flows to the west, ending in the North Sea. The middle part of the Eider had been used for the Kiel Canal.

In the early medieval the Eider was the border between the Saxons and the Danes, as Adam of Bremen reported in 1076. Today it is the border between the two parts of Schleswig-Holstein, Schleswig and Holstein.

The estuary has tidal flats and brackish water.