Mystus armiger, a new species of estuarine bagrid catfish, is described from the mouth of the Kelantan River in the northeastern part of the Malay Peninsula. Mystus armiger resembles Mystus wolffii, both species easily distinguished from congeners by their uniform color without any dark markings, approximately equal lengths of adipose- and anal-fin bases, and long maxillary barbels that reach at least to the base of the caudal fin. Mystus armiger differs from M. wolffii in having a broader snout, smaller eye (17.5–22.8% HL vs 22.8–27.0), and fewer larger serrations on the pectoral spine.