A taxonomic treatment based on field studies, examination of herbarium collections, and previously published molecular data is provided for the North American species of the Sphagnum subsecundum complex. Sphagnum platyphyllum, S. contortum, S. lescurii, and S. subsecundum are gametophytically haploid. Sphagnum missouricum and S. carolinianum are gametophytically allodiploid. A single allodiploid population of S. platyphyllum and a homoploid (haploid) hybrid between S. contortum and S. subscundum, previously documented genetically, are described and illustrated (but neither distinguished taxonomically). Epitypes are designated from (haploid) S. lescurii and (allodiploid) S. missouricum because the ploidal level of the original type material cannot be determined unambiguously. In North America, S. subsecundum is restricted to the eastern part of the continent, as western plants named this species are not conspecific. North American plants named S. inundatum are considered synonymous with either S. lescurii (when haploid) or S. missouricum (when diploid).