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. It is a left tributary of the Elbe near Winsen (Luhe).

The Ilmenau is 121 kilometres (75 mi) long, including its source river Stederau. The largest towns along the Ilmenau are Uelzen and Lüneburg. The city Ilmenau in Thuringia is not along the river Ilmenau but along the river Ilm. The river is navigable from Lüneburg, but almost exclusively used by excursion ships. Above Lüneburg, the river is popular with canoeists.

• List of rivers of Lower Saxony