The rare Baja California Sur endemic cactus Grusonia robertsii was originally hypothesized to be an intergeneric hybrid between G. invicta and Cylindropuntia alcahes subsp. alcahes but was described as a Grusonia due to its overall closer resemblance to G. invicta, except in the fruit. A more comprehensive analysis of the morphology and phylogenetic placement of G. robertsii based on plastid and nrDNA sequences has revealed that Rebman's original hypothesis was correct, and this taxon represents the first documented intergeneric hybrid between Cylindropuntia and Grusonia, with G. invicta as the maternal parent and C. alcahes subsp. alcahes as the paternal contributor. We here describe a new nothogenus, ×Cylindronia, and provide a new combination for the nothospecies ×Cylindronia robertsii comb. nov.