Much attention has been focused on the moss family Mniaceae over the past 50 years, yet new discoveries continue to be made. Plagiomnium decursivum joins P. floridanum as the second new species of Plagiomnium Section Plagiomnium described in the past ten years. Careful morphological and anatomical studies, coupled with evidence from isozymes and gene sequencing, reveal that a species from Japan referred to in an earlier paper (Wyatt & Odrzykoski 1998) as “P. acutum” is actually new to science. In addition, the taxon referred to in that paper as “P. acutum-J” is in fact P. acutum, and the taxon referred to as “P. cuspidatum-F” is now named P. floridanum (Wyatt & Odrzykoski 2012). The new species, P. decursivum, is compared to its closest relative, P. acutum, and to the other four species of Section Plagiomnium. It differs in having smaller leaves with long, broad decurrencies, longer leaf cells with corner thickenings, and shorter teeth composed of a single cell. Based on isozyme data, it is concluded that allopolyploid P. cuspidatum has P. decursivum as one haploid progenitor and that P. floridanum is most likely the other. Moreover, sequencing of two regions of chloroplast DNA (rps4 and rpL16) suggests that P. floridanum may have been the maternal parent in crosses that launched the allopolyploid. The relative isolation of P. drummondii and P. japonicum within the section is apparent from both morphology and genetics.