Hemigrammus machadoi can be distinguished from most congeners by its wide dark horizontal stripe across the eye (vs. eye stripe absent or, when present, vertical, except H. barrigonae, H. lunatus, and H. ulreyi and by its well defined narrow dark stripe at the basis of the anal fin (vs. dark stripe at the basis of anal fin absent, except H. barrigonae, H. boesemani, H. lunatus, H. mimus, and H. ulreyi). It differs from H. boesemani, H. geisleri, and H. mimus by having a distinct dark humeral blotch (vs. humeral blotch absent) and by the absence of a blotch on caudal peduncle or any distinct patch of pigmentation on caudal fin (vs. dark blotch on caudal peduncle present in H. boesemani and H. geisleri, a dark marking present at the basis of each caudal-fin lobe basis in H. mimus); from H. barrigonae and H. ulreyi by absence of a narrow, well-defined longitudinal midlateral dark stripe (vs. present) and by the absence of a discrete blotch on caudal peduncle (vs. present in H. barrigonae) and a dark pigmentation patch on the basis of anteriormost dorsal-fin rays (vs. present in H. ulreyi). It can be distinguished from Hemigrammus lunatus the following characters: a conspicuous vertically elongated dark humeral blotch, extending horizontally from second through sixth lateral-line scales, and vertically from third row above lateral line to first row below it (vs. a small roundish humeral blotch, extending horizontally from fourth through sixth lateral-line scales, and vertically from fourth through fifth scale rows above lateral line); 4-5 gill-rakers on upper branch and 9-10 on lower (vs. 6-7, and 11-12, respectively); the higher number of cusps on inner premaxillary, dentary, and maxillary largest tooth (5-7 cusps, mode 7, in premaxillary and dentary, vs. 5; 3-5 cusps, mode 5, in maxillary vs. 1-3); and 34-35 total vertebrae (vs. 32-33) (Ref. 96864).

Description: Dorsal-fin rays ii,9; anal-fin rays iv, 22-26; pectoral-fin rays i,10-12; pelvic-fin rays i,7; 3-4 scales rows between lateral line and pelvic-fin insertion (Ref. 96864).