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Both sexes of a new brackish-water species, Nannopus sinusalbi sp. nov. (Nannopodidae) are described from the Bahía Blanca estuary (38°53’S, 62°07’W) in Buenos Aires Province, Argentina. The only previous record of the genus in the study area was identified as the type species, Nannopus palustris Brady, 1880, with no description or illustrations, hence its authenticity cannot be confirmed. Nannopus brasiliensis Jakobi, 1956 is relegated to species inquirenda in the genus rather than being considered a junior synonym of the type species. Nomenclatural issues related to the usage of the alternative spellings Iliophilus Lilljeborg, 1902 and Ilyophilus sensu Sars (1909) and the unavailability of Ilyophilus canui Kim, Choi & Yoon, 2017 are discussed. An updated key to the 18 identifiable species of Nannopus (excluding the type species N. palustris) is presented. The harpacticoid assemblage at the type locality showed a distinct seasonality with N. sinusalbi sp. nov. representing about 8% of the community. The new species showed densities below 5 ind.cm‑2 during most of the year, reaching an abrupt peak of 40.17 ind.cm‑2 towards the end of the summer, when the maximum proportion of ovigerous females was recorded.