Nucleotide sequences for six nuclear loci and one chloroplast region were used to reconstruct phylogenetic relationships in Sphagnum section Acutifolia. The combined data matrix, which includes 136 accessions (129 ingroup taxa and seven outgroups) and 5126 nucleotide sites, was analyzed using Bayesian inference. Most of the individual morphospecies commonly recognized in the section were represented by multiple populations, in some cases by up to 16 accessions from throughout the Northern Hemisphere. Results of the combined seven-locus analysis resolved many of the species as monophyletic, but the deeper nodes were generally without support. Separate analyses of single-locus data sets revealed significant conflicts, indicating gene flow among both closely and more distantly related species within the section. The sequence data allowed likely parentage to be identified for several species of hybrid origin, and identified individual accessions that appear to be genetic admixtures. Taxonomic conclusions that can be made from the analyses include: 1. Sphagnum wulfianum and S. aongstroemii should both be included in section Acutifolia, 2. S. subtile cannot be separated from S. capillifolium, and the two should be synonymized, 3. S. capillifolium and S. rubellum each contain a monophyletic core of populations and should be retained as separate species, but 4. S. rubellum cannot be separated from S. andersonianum and S. bartlettianum and the three should be merged, 5. S. tenerum is highly differentiated from S. capillifolium and should be treated as a separate species, 6. interspecific mixed ancestry is demonstrated for S. russowii (a likely allopolyploid), S. skyense , S. arcticum, and S. olafii. Interspecific recombination appears to be rather common in section Acutifolia, yet species, for the most part, maintain cohesiveness.