Hexenaltar, Granite rock formation on Brocken Mountain, Harz, Germany.

The Hexenaltar is a rock formation on the summit of the Brocken in the Harz, within the core zone of Harz National Park. It belongs to the Harzklippen and stands next to the Teufelskanzel, another rock formation. The top stone slab is missing, and the formation is visible from the Brocken summit but not accessible by paths.

During the Kingdom of Westphalia (1807–1813), public newspapers reported that the Hexenaltar had been destroyed by barbarians. In fact, only the top stone slab had been pushed down out of vandalism, and the rock group itself remained intact. An 1844 mention records a witches' dance floor beneath the Hexenaltar.

The formation connects to local legends about gatherings during Walpurgis Night, shaping how people view this place today. These stories draw visitors interested in the folklore traditions of the region.

The site lies within the core zone of Harz National Park and is currently closed to the public. This closure protects the natural environment and allows the area to regenerate.

Heinrich Pröhle recorded a legend that on 1 May brooms, dogs and cats are seen at the Hexenaltar and adults dance there with torches. Another tale holds that an underground passage runs beneath the rock, from which a light called Kobolz sometimes emerges.