A new species of Scleropodium (Brachytheciaceae) is described from western North America. Initially recognized in a recent molecular phylogenetic study of the genus, Scleropodium occidentale B.E. Carter sp. nov. is very similar in morphology, ecology, and distribution to S. obtusifolium. A morphological investigation of S. obtusifolium and S. occidentale was conducted using 62 specimens assigned to species independently based on analyses of DNA sequences of nuclear ribosomal ITS and the chloroplast trnG region. Morphological differences between the two species are subtle, but include relatively robust and commonly spine-tipped stem and branch leaf costae in S. occidentale versus weaker and without a spine in S. obtusifolium, slightly longer lamina cells in S. obtusifolium than in S. occidentale, and a strangulate capsule in S. occidentale in contrast to the cylindrical capsule in S. obtusifolium. Fieldwork and herbarium study indicate that S. occidentale is common in California and is present in Oregon, Washington, Nevada, and southern British Columbia. All herbarium specimens of S. occidentale were previously labeled as S. obtusifolium.