No unique diagnostic character was found for this species, with a combination of the following

characters diagnosing it from all congeners: body depth 43.0–43.2% SL, 15 pectoral-fin rays, 29–32 lateral line scales, 11–12 scale rows below the lateral line, and slightly concave predorsal profile. Among Sundaic Southeast Asian congeners, Nandus mercatus is distinguished from N. nebulosus in having a slightly concave

(vs. even or slightly convex) predorsal profile, rostral fossa reaching to almost half (vs. more than half) predorsal distance and fewer pectoral fin rays (15 vs. 17–19), and from N. prolixus in having fewer lateral-line

scales (29–32 vs. 33–37) and deeper body (43.0–43.2% SL vs. 37.6–40.5). It differs from South Asian congeners N. andrewi and N. nandus in having fewer lateral line scales (29–32 vs. 42–55), fewer scale rows below the lateral line (11–12 vs. 14–18), and deeper body (43.0–43.2% SL vs. 24.3–37.1). Nandus mercatus is distinguished from N. oxyrhynchus in having fewer lateral-line scales (29–32 vs. 37–42) and a more gently-sloping

predorsal profile.