A new species of clover is described, allied to Trifolium fucatum Lindl. and T. depauperatum Desv. Trifolium piorkowskii Rand. Morgan & A. L. Barber (Fabaceae, Papilionoideae) is an extremely rare clover that is apparently endemic to Shasta County, California, U.S.A. This new species differs from all other members of the T. fucatum complex in calyx and floral morphology, leaflet shape and markings, foliaceous stipules, and smooth seed coat. It also differs in range and habitat and has an eccentric nuclear ribosomal DNA ITS profile. Trifolium piorkowskii differs from the T. depauperatum complex in its relative robustness, leaf shape and markings, larger corolla size, the large acuminate involucre lobes, consistently forked calyx teeth, and larger seeds.