Trillium tennesseense is described as a newly discovered member of Trillium subgen. Phyllantherum. The new species resembles the recently described T. oostingii but differs from it by having a shorter ovary, relatively longer staminal filaments and stigmas, and crested-undulate stigmatic surfaces. A chromosome count of 2n = 10 was obtained for it, which agrees with other members of the genus and subgenus. Sequence data from the nuclear ribosomal internal transcribed spacer and the plastid trnH-psbA spacer regions provided further evidence of the distinctiveness of T. tennesseense. A phylogeny estimate based on these markers is presented for T. subgen. Phyllantherum which agrees well with the major species groups that have been previously recognized in the subgenus. Trillium tennesseense and T. oostingii were placed in the molecular-based phylogeny in a clade with T. lancifolium and T. recurvatum, two species that formed a previously recognized species group. Trillium tennesseense is currently known only from three locations, all occurring on the Sevier Shale formation on the northwest slope of the southeastern extension of Bays Mountain in Hamblen and Hawkins Counties, Tennessee. Based on its rarity, T. tennesseense should be considered as globally rare and in need of protection.