Hydrothermal vents near the summits of seamounts in Western Pacific volcanic arcs foster dense populations of flatfishes, a group otherwise unknown at vents. We examined genetic divergence among populations of a symphurine tonguefish described as Symphurus thermophilus Munroe & Hashimoto from sites up to 6000 km apart in the Western Pacific to explore connectivity patterns among seamounts. Average genetic divergence between individuals from the Mariana Arc and the Tonga-Kermadec Arcs was 14.2% (COI) and 9.0% (16S), whereas within-arc divergences were <0.3%. We found that the Tonga-Kermadec individuals represent a cryptic species, Symphurus sp. A, displaying similar phenotypic features and behaviour to the Mariana Arc S. thermophilus. We also sequenced another distinctive symphurine species; Symphurus sp. B. Collections and image records from three expeditions to the Tonga and Kermadec arcs revealed characteristics of the distribution, dispersion, behaviour and morphology of these flatfish species. These two new Symphurus species inhabit vents where native sulphur occurs in excess and depths are <600 m. Substratum range of Symphurus sp. A was wide, including mussel beds, rock surfaces and sediments in densities that may exceed 100 m−2. Reproductive females were present. The complex and diverse nature of volcanic settings of hydrothermalism introduces a wide variety of habitat conditions that likely augments diversity of faunas on seamounts in distinct biogeographic provinces.