Submarine hydrothermal vents and associated seafloor mineralization on the Tonga arc have been found for the first time, in the summit calderas of two shallow-water volcanoes, greatly extending the known areas and diversity of seafloor hydrothermal activity in the western Pacific region. The highest temperature vents (245–265 °C) occur at water depths of 385–540 m near the summit of one volcano at 24°S. The vents are spatially related to basaltic dike swarms exposed at a summit cone and in the caldera walls. Clusters of large (to 10 m high) barite, anhydrite, and sulfide chimneys on the summit cone are vigorously discharging clear hydrothermal fluids with temperatures on the seawater boiling curve. There is abundant evidence of phase separation, which can be seen as flame-like jets of steam (H2O vapor) at the chimney orifices. Pyrite, marcasite, sphalerite, and chalcopyrite line the interiors of the highest temperature vents, similar to black smoker chimneys on the mid-ocean ridges.