Fens that formed over acidic geothermal springs in Yellowstone National Park support species of Sphagnum previously unknown or rare in the Rocky Mountain region of the US. These taxa are more common in boreal regions of North America. During our research on the biodiversity of fens in Yellowstone we identified four species of Sphagnum new to the state of Wyoming: Sphagnum lindbergii Schimp. in Lindb., Sphagnum riparium Ångstr., Sphagnum fimbriatum Wils. & Hook. in Hook., and Sphagnum capillifolium (Ehrh.) Hedw. Acidic geothermal fens are similar to iron fens reported in other parts of the Rocky Mountains; they support many rare and disjunct Sphagnum species. Acid geothermal fens and iron fens appear to function as links between floras of the southern Rocky Mountains and boreal regions.