Leporinus amazonicus belongs to the friderici-group, as defined by Géry (1977) and

Garavello (1979). It differs from most of congeners (except L. fasciatus, L. affinis and L. y-ophorus) by the high lateral line scale count (45–47 versus 43 or fewer), and from these species by the color pattern (brownish- grey with one small and elongated dark blotch on the flanks in L. amazonicus vs. yellow ground color with

several single or divided vertical dark bars on the side in the others). Leporinus amazonicus is morphologically similar to the slender bodied species L. cylindriformis Borodin, 1929 and L. brunneus Myers, 1950, but differs from them by the higher lateral line scale count (45–47 versus 41–43 in L. cylindriformis and 36–42 in L. brunneus) and by a different color pattern (lateral blotch occupying 2–4 horizontal scales in L. amazonicus versus 5–7 in L. cylindriformis, and absence of well defined blotches on the body sides in L. brunneus) (Ref. 76907).