"Trillium grandiflorum" was originally described as "T. rhomboideum" var. "grandiflorum" by André Michaux in his 1803 work "Flora Boreali-Americana". "T. grandiflorum" was first treated as a species by Richard Anthony Salisbury only two years later. The species was traditionally placed in the subgenus "Trillium", but recent work has shown the phylogeny of this grouping to be paraphyletic. Since then no phylogeny of the genus has garnered much agreement. (The other subgenus, "Phyllantherum", includes sessile-flowered species.) "T. grandiflorum" is, alongside the western "T. ovatum", one of the closer relatives to the subg. "Phyllantherum" members.