Dendrobium petrophilum (Kraenzl.) Garay ex N.Hallé was a species only known from the type specimen and was considered doubtful. Since it has been recently relocated in its type locality and in its surrounding areas, we were able to compare it to the endemic closely related and well known species, Dendrobium oppositifolium (Kraenzl.) N.Hallé. We confirm D. petrophilum as a distinct species based on its morphology and its very special ecology. Both species are fully described, compared, and illustrated. Comparative tables are given in English and French. We also discuss their ecology, distribution, phenology and confirm the IUCN assessments made by the local Endemia Red List Authority.