Bagrichthys majusculus, a new species of bagrid catfish from Indochina, is very similar to B. macracanthus and B. vaillantii, and has been previously identified as the former species. It differs from congeners in having a unique combination of the following characters: relatively large and broad mouth, well-developed oral dentition with homodont teeth, 10–13 gill rakers, moderately-long dorsal spine with 15–27 serrations, 9 pectoral-fin rays, inner and outer mandibular barbels with straight margins, pectoral-spine length 15.8–20.7 % SL (standard length), dorsal-spine length 24.4–32.5 % SL, length of adipose-fin base 46.0–50.7 % SL, adipose maximum height 9.9–10.5 % SL, depth of caudal peduncle 7.1–7.5 % SL, and head depth 14.0–16.1 % SL.