Corydoras knaacki is distinguished from its congeners by having the following combination of characters: a longitudinal black stripe along midline of flank; short mesethmoid, with anterior portion smaller than 50% of the bone length; serrations on posterior margin of pectoral spine directed towards the tip of the spine; dorsal fin with the region of the first branched ray, including membrane, with black pigmentation, the remaining areas with irregular black blotches; lacking a vertically elongated black blotch across the eyes; snout with conspicuously rounded moderately-developed black spots; and ventral expansion of infraorbital 1 moderately developed (Ref. 121196).