The high temperatures of sea-floor hydrothermal vents make them good targets for the exploitation of thermal energy. Taking advantage of this prospect, this study developed a thermoelectric converter that harvests thermal energy from hydrothermal fluids through a heat pipe and converts heat to electrical energy with thermoelectric generators. A power management system that enables the thermoelectric converter to continuously power a data logger and a light-emitting diode lamp was also proposed. The thermoelectric converter was field tested at a deep-sea hydrothermal vent with a depth of 2765m at the Dragon Flag Field along the Southwest Indian Ridge. With the use of the thermal gradient between hydrothermal fluids and seawater, the thermoelectric converter obtained a sustained power of 2.6–3.9W during the field test. Our results demonstrate that the thermal energy of hydrothermal fluids can be an alternative renewable power source for seabed observation equipment that requires watt-level power.