Kleine Harzburg, Medieval castle ruin in Bad Harzburg, Germany.

Kleine Harzburg is a castle ruin perched on a hilltop near Bad Harzburg, with surviving foundation walls, tower remnants, and a moat that divided the structure into two sections. The remains clearly show the layout and fortifications of the medieval fortress.

No historical record names the Kleine Harzburg directly, so its dating and role remain debated. The most plausible reading is that it served as an outwork of the Große Harzburg, built after 1068 under Henry IV, whose building techniques and forms it matches, which would give it imperial castle status. Finds point to a short lifespan; archaeologist Wolf-Dieter Steinmetz assumes the site was abandoned after the Große Harzburg was temporarily given up in 1076. No artefacts can be tied to the Große Harzburg's use under Otto IV in the early 13th century.

No document names the site or explains its use, so its role is inferred: most likely a forward bastion of the Große Harzburg, sharing its building technique and form. The name Kleine Harzburg simply sets it apart from its larger neighbour 300 m to the south. Finds point to a short life, and archaeologist Wolf-Dieter Steinmetz suggests it lay abandoned after the Große Harzburg was temporarily given up in 1076.

The ruin lies on the Kleiner Burgberg, about 300 m north of the Große Harzburg; the approach is from the southwest, where no gate structure is discernible. A neck ditch 15–20 m wide cuts the ridge south of the main castle, and a second one encloses a roughly 30 m wide outer bailey. The terrain is uneven, with two cellar depressions and a cistern still visible.

No artefacts from Otto IV's use of the Great Harzburg in the early 13th century can be assigned to the Kleine Harzburg, so the two sites are not linked by finds. Archaeologist Wolf-Dieter Steinmetz instead suggests the castle was abandoned after the Great Harzburg was temporarily given up in 1076. The well beside the northern building was cleared in 1905 to a depth of 32 metres.