The North American vinegaroon, Mastigoproctus giganteus (Lucas, 1835), is demonstrated to comprise a complex of range-restricted species rather than a single widespread polymorphic species. Seven species are recognized based on morphological characters of the adult males, including the arrangement of spines on the prodorsal margin of the pedipalp trochanter, the position of the epistoma on the carapace, the presence of a stridulatory organ on opposing surfaces of the chelicerae and the pedipalp coxa, the presence of a patch of setae on sternite V, and the shape and macrosculpture of the retrolateral surface of the pedipalp femur. The two currently recognized subspecies are elevated to species: Mastigoproctus mexicanus (Butler, 1872), stat. nov., and Mastigoproctus scabrosus (Pocock, 1902), stat. nov. Mastigoproctus floridanus (Lönnberg, 1897) is revalidated from synonymy with M. giganteus. Redescriptions of M. giganteus and the other three species, based on both sexes, are provided, and three new species described: Mastigoproctus cinteotl, sp. nov., from Tamaulipas, Mexico; Mastigoproctus tohono, sp. nov., from Arizona and Sonora, Mexico; Mastigoproctus vandevenderi, sp. nov., from Sonora, Mexico. The present contribution raises the diversity of the Order Thelyphonida Latreille, 1804, in North America from one species to seven. Three species occur in the United States (one each in Arizona, Texas, and Florida), six species occur in Mexico, and two species occur in both countries.