Exobasidium pentasporium was first found on Rhododendron kaempferi in Nikko, Tochigi Prefecture, Japan and described only with a brief mentions and illustration of a specimen in 1896. This fungus causes a witches' broom disease of Rhododendron species. To stabilize the concept of this species, the specimen in the protologue was located, carefully examined, and illustrated. In addition, the name was epitypified based on a newly collected topotype specimen. A phylogenetic tree using ITS and LSU sequences showed that our isolates of E. pentasporium grouped with other Exobasidium species on Rhododendron forming a monophyletic clade with strong statistical support and were unrelated to E. nobeyamense, another causal agent of witches' broom disease on Rhododendron species.