Imparfinis mishky sp. n. is described from the río Paraná and río Uruguay basins in Argentina. Imparfinis mishky sp. n. is distinguished from other species of the genus by the following combination of characters: pectoral-fin spine with smooth anterior and posterior margins; caudal fin deeply forked with dorsal lobe longer than ventral lobe; adipose fin not reaching caudal fin; body with six dark saddles across the dorsum, and without a mid-lateral band. It was found inhabiting well oxygenated, neutral pH, flowing waters, with sandy or cobble covered bottom.