Technical Abstract: The fungus causing annular leaf spot of potato has been reported only from South America, and poses a quarantine risk as a potentially invasive species due to its ability to flourish in cooler temperatures typical of potato-producing regions in Europe and North America. It was initially described as a variety of Septoria lycopersici, but this taxon, “Septoria lycopersici var. malagutii”, was not validly published because the type specimen was not specifically designated as required by the International Code of Botanical Nomenclature. We examined specimens of Septoria lycopersici var. lycopersici and “S. lycopersici var. malagutii” and sequenced the internal transcribed spacers (ITS) of the ribosomal RNA and the translation elongation factor (TEF)-1' gene. Conidia of “S. lycopersici var. malagutii” were significantly longer and narrower and conidiogenous cells were significantly shorter and narrower than those of S. lycopersici var. lycopersici. Although the ITS and TEF-1' sequences were similar (99.6% and 99.2%, respectively), the ITS of S. lycopersici var. lycopersici was more similar to several other Septoria species than to “S. lycopersici var. malagutii” suggesting that this entity should be recognized as a distinct species. Based on the morphological and molecular sequence differences, we validate and redescribe as a new species Septoria malagutii Ciccar. & Boerema ex E.T. Cline, sp. nov.