We present the first record of Potamomyces Hyde (Ascomycota, Dothideomycetidae) from soil sediments of the El Palmar National Park, Entre Ríos, Argentina. Nineteen samples were taken (each 2–4 cm from the surface) of a 80 cm thick core composed of a homogeneous lithology of argillaceous fine-grained sands. This taxon was only identified in the basal sample of the core (371 ± 30 years BP), located in a palm area (Butia yatay) with a semi-dense herbaceous-shrub layer. We propose an amendment of the generic diagnosis based on the revision of the species described up to now, and of a new one here defined (P. palmarensis). Additionally, we confirm that the morphotaxon Mediaverrunites Jarzen & Elsik ex Nandi & A. Sinha is its junior synonym, so we propose new combinations for seven species. We recommend the use of this natural taxon instead of the morphotaxon in future contributions. In terms of the geographical and stratigraphical distribution of Potamomyces, our record is the first mentioned for Argentina and the southernmost for South America. Based on its ecological requirements, a humid sub-tropical environment and abundant organic matter in the soil are suggested for the Latest Holocene of the study region. This hypothesis is supported by current knowledge of regional climatic and environmental conditions for the study area during the short humid interval of the Little Ice Age.