Characidium samurai is distinguished from all its congeners, except C. lanei Travassos, by having a dark lateral band along the head and body that is particularly broad (1.5 or 2 scales wide) from the rear of head to the end of the caudal peduncle and by the absence of dark bars or blotches on the ventral half of the body. It can be diagnosed from C. lanei by having the lateral band with straight borders overall (vs. lateral band with somewhat irregular borders due to blotches slightly extending dorsally or ventrally), anal fin ii,7-8 (vs. ii,6), 4 horizontal scale rows above the lateral line and 4 below (vs. 5/3), and lacking dark pigmentation around the insertion of the dorsal-fin rays (vs. presence). It further differs from congeners that occur in Brazilian rivers by having a complete lateral line, isthmus completely covered by scales, 14 scales around the caudal peduncle, and presence of adipose fin. It can be further distinguished from congeners by the absence of dark bars or spots on the fins, except for a faded dorsal-fin bar and anal-fin rays ii,7-8. It differs from congeners from northeastern Brazilian rivers by having the following characters: aperture of the pseudotympanum enclosed between ribs of the fifth and sixth vertebrae; presence of the parietal branch of the supraorbital canal; two series of teeth on the dentary; and 7-11 premaxillary teeth; 7-8 branched anal-fin rays; 34-37 lateral-line scales; one faded dark bar on the proximal half of the dorsal-fin rays; absence of a blotch on the caudal peduncle, or blotch merged to the longitudinal bar; and ii,7-8 anal-fin rays (Ref. 96872).