Opuntia, the most widespread genus of cacti, occurs throughout the Americas from Patagonia to Canada. Various species have very wide distributions in the Americas, and thus may be considered as both native and aliens. We reexamined data based on recent work on the phylogenetics and taxonomy of Opuntia from southern S-America and showed that two presumed endemic species to Argentina—O. penicilligera and O. ventanensis—are likely derived from, or may be conspecific, with North American species. In particular, O. penicilligera is most closely related to members of the O. macrorhiza species complex and is morphologically similar to both O. macrorhiza and O. cymochila. O. ventanensis shares nearly all vegetative and reproductive morphological characters with O. fragilis and likely is conspecific with that taxon. Owed to the wide distribution of Opuntia species and the movement of many taxa by people, extra care must be exercised when describing new species or for carrying out taxonomic treatments. A phylogenetic perspective, as well as a careful study of species across the distribution of the genus, is recommended.