A new species, Antrophyum nambanense, is described here based on the evidence from morphology and molecular phylogeny. Morphologically, short taeniform soral paraphyses and large fronds are the diagnostic characteristics of A. nambanense. Phylogenetically, it is most closely related to A. annamense, but can be readily distinguished from A. annamense by taeniform soral paraphyses (vs. filiform). We suggest A. nambanense is an allopolyploid of hybrid origin involving A. annamense and A. henryi, based on the evidence of morphology, phylogenetic position, chromosome number, and partially malformed spores. A description, photographs of key characters, distribution, conservation status, and ecology of the new species are presented. In addition, A. nambanense is also the first confirmed species with chlorophyllous spores in Antrophyum. Lastly, an updated key to all known Antrophyum species in Vietnam, including A. henryi as a new record for the country, is provided.