The American species of Trifolium section Lupinaster are reviewed. A key to the species, descriptions of species which have not been published already in previous papers (Gillett 1965, 1969, 1971, 1972), pertinent synonyms, typification, and distribution maps and notes are provided. The chromosome number 2n = 16 is reported as new for Trifolium douglasii House. A new subspecies, T. andersonii Gray subsp. beatleyae Gillett, is described and the following combinations are made: T. plumosum Douglas subsp. amplifolium (Martin) Gillett, T. gymnocarpon Nuttall subsp. plummerae (Watson) Gillett, T. andersonii Gray subsp. monoense (Greene) Gillett, and T. kingii Watson subsp. macilentum (Greene) Gillett. Morphological relationships are discussed.