Chateau Bû is a Neolithic chambered burial cairn that forms part of the St Just complex. It is highly atypical, topped with a small circle of stones and four large menhirs (one toppled). The cairn may have been subject to restoration work, but information is difficult to come by.

In 'Megalithic Brittany', Aubrey Burl tells us: "Tradition has it that each year in ancient times a young girl was sacrificed on the mound on an altar specially built for the occasion. The meaning of Chateau Bû is obscure, but Bû may derive from the Breton word for cattle, bulls having an important role in the religious celebrations of Brittany."

The cairn is currently fenced off and surrounded by thick vegetation. Viewing is only possible from a few angles. Nevertheless, this is easily one of the more fascinating and visually striking monuments at the St. Just complex, and very much worth a visit.