Brachychalcinus reisi is distinguished from all other species of the genus by having a series of dark, conspicuous, wavy longitudinal stripes on the entire body (vs. longitudinal dark wavy stripes absent or restricted to middorsal portion of body) and by having 7-8 longitudinal scale rows between dorsal-fin origin and lateral line (vs. 8-12). In addition, it is diagnosed from B. copei (Steindachner, 1883), B. retrospina Boulenger, 1892 and B. parnaibae by having 9 branched dorsal-fin rays (vs. 10-11). It can be differentiated from B. nummus Böhlke, 1958 and B. orbicularis (Valenciennes, 1850) by having 8-9 horizontal scale rows between the lateral line and pelvic-fin origin (vs. 10-11). It further differs from B. nummus by having numerous paralleled arranged scale radii (vs. few divergent arranged scale radii) (Ref. 118114).