A new species of Sphagnum section Subsecunda, S. beringiense , is described from arctic Alaska from the vicinity of Barrow along the northern coast. The species is distinguished morphologically by the light, yellow-green color of the gametophytes, multistratose stem cortex with 2–4 layers of enlarged thin-walled cells, round, medium-size (ca. 5 μm diameter) outer branch leaf pores, scattered inner branch leaf pores, typically few outer stem leaf pores, and abundant, round to elliptic inner stem leaf pores. Two unique plastid DNA haplotypes occur among Barrow area plants of S. beringiense , and these differ by a minimum of three nucleotide substitutions from those of other Alaskan Sphagnum species in the section Subsecunda. Microsatellite markers show that S. beringiense is genetically variable despite the fact that all plants were sampled from within an area of a few km² and neither gametangia nor sporophytes have been observed. A key to the six Alaskan species of Sphagnum section Subsecunda is provided.