Ptychostomum pacificum J. R. Spence & Shevock, a new and highly distinctive species restricted to fen habitats within coniferous forests in California, Oregon, and extreme western Nevada is described and illustrated. This species appears to be related to P. turbinatum (Hedw.) J. R. Spence but is easily distinguished by a combination of features including large size, percurrent yellowish-brown costa with numerous incrassate cells at the leaf tip, strongly recurved leaf margins, and elongate cylindrical to narrowly pyriform capsules.