Sieber is a river of Lower Saxony, Germany, in the Harz mountains.

In February 1910, as part of his hydrological research into the Rhumequelle, Karl Thürnau dyed the Sieber tributary of the Eichelbach (Herzberg) with six kilogrammes of fluorescein.

Three days later, weakly dyed water appeared in the main source of the Rhumequelle.

This was confirmed in 1980 by dye tracing carried out by the Geological Survey of Lower Saxony (Niedersächsischen Landesamtes für Bodenforschung).

The upper Sieber valley has been out-of-bounds to motor traffic since the 1980s from the Forsthaus Königshof and lies partly within the Harz National Park – the road leads to the B 242 federal highway northwest of Sonnenberg.