Potamarius usumacintae, new species, is described based on specimens collected in the Río Usumacinta basin in Guatemala and México. The species is diagnosed from other freshwater ariids distributed in Mesoamerica by the following combination of features: lateral gill rakers on first gill arch 17–20 (mode 19); mesial gill rakers on first two gill arches well developed; accessory tooth patches on palate present; premaxillary tooth band length 2.4–2.9 in its width; snout length 1.4–1.8 in postorbital length; maxillary barbel not extending behind gill membrane; dorsomedian fleshy groove on head absent. A key to identify nine Atlantic slope ariid species from North and Central America is provided.