Ferocactus wislizeni subsp. ajoensis is described as a new subspecies, and documented from the counties of Pima, Pinal, Maricopa and Yuma, Arizona, USA. It is distinguished from the type subspecies of F. wislizeni by its slender stem proportions, longer, denser and more robust spines and earlier and longer flowering period during the summer months. Unlike the type subsp. wislizeni that is found mostly on sandy and rocky soils, the characteristic habitats of subsp. ajoensis are the flat basin floors dominated by silts and clays.