A new endemic moss from California is described and illustrated. The heterophyllous leaves and weakly cladocarpous growth pattern in crescent shaped cushions are distinctive features for this species. Although morphologically distinct from Orthotrichum, phylogenetic inferences based on variation in nucleotide sequences of the rbcL gene (cpDNA) suggest this species to be a member of Orthotrichum. Review of Orthotrichum specimens from California herbaria concluded that this species had not been collected previously. Orthotrichum kellmanii is currently restricted to a narrow band of sandstone outcrops on ridges overlooking the Pacific Ocean.