Hyphessobrycon kayabi can be diagnosed from all its congeners, except for H. bifasciatus, H. cyanotaenia, H. heliacus, H. igneus, H. loweae and H. peugeoti by having distal border of the anal fin slightly convex to straight, without a distinct anterior lobe in adult males, with last unbranched ray and first to second branched ray shorter than the subsequent branched rays (vs. distinct anterior lobe present in most congeners, with last unbranched ray and first and second branched rays longer than remaining rays). It differs from H. bifasciatus, H. cyanotaenia, H. heliacus, H. igneus, H. loweae, and H. peugeoti by having octa- to decacuspid inner premaxillary teeth (vs. tetra- to pentacuspid in H. bifasciatus and H. igneus, heptacuspid in H. heliacus, penta- to heptacuspid in H. cyanotaenia and H. loweae, and hexa- to heptacuspid in H. peugeoti), and by having dark chromatophores concentrated on the posterior border of the body scales, forming a reticulated color pattern (vs. reticulated pattern absent). It can be separated from Hyphessobrycon heliacus, H. loweae, and H. peugeoti by the absence of an elongated, filamentous dorsal fin in adult males (vs. elongated, filamentous dorsal fin present in adult males). It further differs from H. bifasciatus by having a single humeral blotch (vs. two well-defined humeral blotches) and from H. bifasciatus and H. igneus by having 17-20 branched anal-fin rays (vs. 27-32 in H. bifasciatus, and 23-29 in H. igneus) and by the absence of hooks on anal fin in mature males (vs. presence in H.bifasciatus and H. igneus) (Ref. 94770).