Astyanacinus yariguies, a new species, is described from Río Cascajales, Río Magdalena system, in the municipality of El Carmen, Santander, Colombia. This species is assigned to Astyanacinus because it exhibits all the diagnostic characters of the genus, including 25–36 branched anal-fin rays (28–30 in the new species), and a coloration pattern with two humeral spots, a midlateral stripe formed by anteriorly oriented chevron bars, and a horizontally oriented oval-shaped caudal spot that continues over the median caudal rays. Astyanacinus yariguies differs from its congeners by having 13 predorsal scales forming a regular series, six teeth in the maxilla, nine scale rows between the dorsal-fin origin and lateral line, and a relatively short snout length. This is the first report of the genus Astyanacinus from the Río Magdalena basin.