The Allerbach is a tributary of the Warme Bode in the Harz Mountains of central Germany. It is just under 5 kilometres (3 mi) long.

The source region of the "Little Allerbach" (Kleiner Allerbach) is in a boggy wood at a height of 560 m above sea level (NN) and lies about 100 metres (330 ft) north of the level crossing on the K 1353 county road (Kreisstraße) (Sorge – Elend section) with the railway line of the Trans-Harz Railway.

The source region of the "Big Allerbach" (Großer Allerbach) is on the southwest hillside of the Rauher Jakob (560 m above NN); it lies at a height of 540 m above sea level (NN).

The stream flows downhill in an easterly direction.

Regarding its confluence immediately next to the L 98 Landesstraße (Tanne – Königshütte section), the Allerbach forms the historic boundary of the Amt of Elbingerode. Even today there are numerous boundary stones left and right of the stream banks.

• List of rivers of Saxony-Anhalt