Osmunda subgenus Osmunda is variable across its worldwide range. In addition to the three species assigned to the subgenus, there is an enigmatic group of plants in Laos and Myanmar. To reveal the taxonomic and evolutionary status of these plants, we performed taxonomic, cytological, and flow-cytometric analyses, and molecular phylogenetic analyses for 32 samples of subgenus Osmunda for three nuclear DNA markers and the chloroplast rbcL gene. Results show that the enigmatic plants are a tetraploid (n = 44, 2n = 88) of hybrid origin from Osmunda regalis and O. japonica, which is described as Osmunda hybrida sp. nov. This is the first record of a naturally occurring polyploid species in the most primitive leptosporangiate family Osmundaceae, previously thought to be diploid throughout. Molecular data suggest that Indian O. regalis is a putative maternal ancestor and O. japonica is a paternal ancestor. Osmunda hybrida shares the fully dimorphic or partially dimorphic leaves with its parental species. Osmunda hybrida presumably originated by interspecific hybridization and polyploidization when the parental species overlapped or less likely via long-distance gametophyte interaction following spore dispersal.