Philonotis breutelioides Shevock & Aguero is described as a new endemic moss species restricted to montane and subalpine fen habitats in California from the southern Cascades and Klamath Range south to the southern Sierra Nevada. Molecular data suggests an affinity within the genus Philonotis even though this new species shares morphological character states more frequently observed in the genus Breutelia. Available molecular data suggest distinctiveness of the new species within Philonotis fontana lineage, but they are insufficient to resolve relationships among the taxa in this lineage. Morphologically, Philonotis breutelioides differs from P. fontana by having more narrowly tapered leaves with slightly flexuose apices, leaf base with obvious plications, and elongate thin-walled cells in more numerous rows between costa and margin.