The Ecker is a 28-kilometre (17 mi), right-hand, southeast tributary of the Oker which runs mainly through the Harz mountains in the German states of Saxony-Anhalt and Lower Saxony.

From its source to Abbenrode the Ecker is a border river, today running between the federal states of Saxony-Anhalt and Lower Saxony.

The Ecker rises around 2.5 kilometres (1.6 mi) southwest of the Brocken at 890 m above sea level (NN) at the Eckersprung.

The upper Ecker valley is part of the Harz National Park.

Before Wiedelah some of the water is diverted as the Ecker Ditch (Eckergraben) and only feeds back into the Oker 10 kilometres (6 mi) north of Schladen.