We describe and illustrate a new firmoss, Phlegmariurus changii (Huperziaceae), which is endemic to eastern Taiwan. This new species is most similar to Phlegmariurus carinatus (Desv. ex Poiret) Ching; however, it differs by leaves that are flat abaxially. In addition, the sporophylls and trophophylls are conspicuously dimorphic for Phlegmariurus changii, but essentially monomorphic in Phlegmariurus carinatus. The ecology, conservation status, and morphology of P. changii is compared to that of species in three other sections (Sect. Phlegmariurus, L. B. Zhang, Sect. Huperzioides H. S. Kung et L. B. Zhang, and Sect. Carinaturus (Herter) H. S. Kung et L. B. Zhang) of Phlegmariurus in East Asia.