Atlantirivulus guanabarensis can be diagnosed from all species of the genus Atlantirivulus by the following combination of characters: lateral line divided into two sections (vs. single); lateral line with one neuromast by scale (vs. lateral line often interrupted by scales without neuromasts); 22 - 26 neuromasts in the section of infraorbital series around orbit (vs. usually 17 - 22, sometimes 23 or 24); distal margin of the anal fin and ventral margin of the caudal fin with a narrow, well-defined dark reddish grey stripe in males (vs. never a similar color pattern), melanophore patch on the postorbital region extending ventrally to the pre-opercular series of neuromasts (vs. not reaching pre-opercular series of neuromasts); snout blunt (vs. slightly pointed); ventral portion of head and venter white (vs. orangish yellow); flank without triangular light blue spots in males (vs. present); and base of the dorsal and anal fins without grey reticulum in males (vs. present) (Ref. 102354).

Description: caudal fin oval; longitudinal series of scales 33- 34; transverse series of scales 7-8 (Ref. 102354).