A new symphurine tonguefish species, Symphurus megasomus, is described on the basis of 18 specimens collected in deep waters (471–640 m) off eastern Taiwan. This species is characterized by the combination of a predominant 1-2-3-2-2 pattern of interdigitation of dorsal pterygiophores and neural spines, 14 caudal-fin rays, 106–111 dorsal-fin rays, 91–96 anal-fin rays, 55–58 total vertebrae, 5 hypurals, deep body (27.2–30.8% SL), small (5.3–7.9% HL), round and separate eyes, bluish-black opercle and peritoneum on both sides of body (in both fresh and alcohol-preserved specimens), uniformly straw-colored to dark-brown ocular-side background pigmentation without freckles or mottled pigmentation, and uniformly white to light yellow blind side. Among congeners, S. megasomus is most similar to S. undatus, which has the same ID pattern and number of caudal-fin rays, but differs markedly from that species in having more dorsal- and anal-fin rays and in its ocular-side coloration (uniformly straw-colored to dark brown vs. freckles on the ocular side of S. undatus).