Ilmenau is a river south of Hamburg, in Lower Saxony, Germany.

The Ilmenau originates in the Lüneburg Heath, south of Uelzen, at the confluence of the rivers Gerdau and Stederau in Veerßen [de], a district of Uelzen.

The Ilmenau is 121 kilometres (75 mi) long, including its source river Stederau.

The river is navigable from Lüneburg, but almost exclusively used by excursion ships.

This article related to a river in Lower Saxony is a stub.