The first suite of oceanic epidosites has been recovered from the Tonga forearc. Epidosites are metal-depleted rocks composed of subequal proportions of epidote and quartz that lack any original igneous texture. Epidosites have been interpreted to form in upflow zones at the base of ore-forming oceanic hydrothermal systems that vent as black smokers on the sea floor. Tongan epidosites metasomatically replaced basaltic and plagiogranite protoliths and formed under similar conditions to epidosites hosted in many ophiolites interpreted to have formed in a suprasubduction-zone setting. The similarities shared by the Tongan and suprasubduction-zone ophiolite epidosites, along with the absence of epidosites in collections from modern mid-ocean ridges, suggest that tectonic setting plays a role in their formation.