This paper clarifies the identity of Cereus macrogonus Salm-Dyck, the type species of the genus Trichocereus (Berger) Riccob., and specifies its synonyms and affinities. It is also intended to contribute toward stabilizing the generic nomenclature. An examination of Salm-Dyck's original description of C. macrogonus leads to an interpretation of the species, very plausibly of Peruvian origin, as a cactus which was later also named Trichocereus peruvianus Britton & Rose. A Neotype is designated that is consistent with the protologue. That interpretation coincides with the use of this name (C. macrogonus) in the botanical literature and in horticulture, which has been adhered to up to now. Trichocereus pachanoi Britton & Rose is here presented as a variety of T. macrogonus (Salm-Dyck) Riccob.: T. macrogonus var. pachanoi (Britton & Rose) S.Albesiano & R.Kiesling, nov. comb., and a lectotype for it is designated.