Wiedau is a river of Lower Saxony, Germany. It is a left-hand tributary of the Wümme.

The Wiedau begins near Bellen (a district of Brockel) in the vicinity of Hemslingen in the district of at the confluence of the Hahnenbach and the Mehlandsbach. In the area the Samtgemeinde Bothel, it flows through a densely wooded and near natural depression. It discharges into the Wümme in Rotenburg shortly after being joined by the only slightly smaller Rodau.

The Wiedau is popular with anglers and canoeists. At high water, the Wiedau can be navigated from the road bridge in Bellen near Hemslingen to its mouth on the Wümme by small kayaks, a distance of 23 km (14 mi).

• List of rivers of Lower Saxony