A new species of Kumanasamuha is described and illustrated from axenic single-spore isolates of Chorioactis geaster. The characteristics of conidia and hyphae are the same as the dematiaceous hyphomycete observed on decayed trunks of Quercus spp., where C. geaster appeared in the field in Japan. Our results indicated that C. geaster in situ colonizes decayed trunks of Quercus spp.