Taxonomic uncertainties regarding rare species often impede effective biodiversity conservation. One such taxonomic uncertainty is the 95-year-old mystery surrounding Arctoa schistioides (Broth. ex Ihsiba) Ihsiba. Since its initial publication in 1929, this species has not been subjected to any further discoveries and is, thus, classified as “doubtful taxa” or “insufficiently known taxa” to date. Assessing the taxonomic status of this species is essential for determining whether a conservation strategy should be implemented. In this study, we examined the holotype of A. schistioides and treated this species as a new synonym of Kiaeria falcata (Hedw.) I.Hagen, a widely distributed species in the Northern Hemisphere, by providing detailed description, illustration and taxonomic notes. Our findings not only resolve this long-standing mystery, but also enhance our understanding of Japanese mosses and the global distribution of bryophytes.