Hyphessobrycon montagi is distinguished from all its congeners, with exception of H. balbus, H. bifasciatus, H. columbianus, H. condotensis, H. flammeus, H. griemi, H. isiri, H. itaparicensis, H. savagei, H. sebastiani, H. togoi, H. tortuguerae, and H. weitzmanorum, by having two well-defined humeral blotches. It differs from all these species by the having the following characters: a well-defined caudal peduncle blotch (vs. caudal peduncle blotch absent in all aforementioned species, with the exception of H. isiri and H. togoi); a horizontally-elongated oval, very well-defined first humeral blotch (vs. first humeral blotch moderately conspicuous, vertically elongated, and approximately rectangular in shape); and first and second humeral blotches are connected via a narrow, short stripe (vs. humeral blotches not connected) (Ref. 97524).

Description: Dorsal-fin rays ii, 9; anal-fin rays iii, 18-22; Pectoral-fin rays i, 9-11; pelvic-fin rays i,7; lateral series scales including lateral line scales 27(2), 28(7), 29(17), 30(19), 31(13), 32(7), 33(9), or 34(7); horizontal scale rows between lateral line and pelvic-fin insertion 3 (Ref. 97524).