Mercury ion (Hg²⁺) is a highly toxic matter that can result in serious dangers to human health. Therefore, monitoring the mercury ion concentration in the environment is an extremely important issue. Sensing approaches like atomic absorption spectrometry and inductively coupled plasma mass spectrometry have been applied to detect the mercury ion. However, the heavy and expensive instruments show the difficulty in performing on-site analysis. In our research, we developed a self-powered mercury ion sensor based on thermoelectric effect to overcome the drawback. Tellurium nanowires were synthesized as the probing and thermal energy harvesting materials. The surface of tellurium nanowires is selective for mercury ion binding. Under a temperature difference, the thermoelectric device based on mercury ion binding tellurium nanowires shows a higher electric output than the tellurium nanowires-based thermoelectric device. The electric output of the as-developed self-powered sensor shows a positive correlation with the mercury ion concentration. In addition, we also tested other common metal ions (Na⁺, K⁺, Cu²⁺, Mg²⁺, Ca²⁺, Ba²⁺ and Zn²⁺) that existed in the environment, confirming that the self-powered sensor is selective for mercury ion detection. Based on those results, we believe that the thermoelectric effect-based self-powered sensor serve as the stepping stone for the development of cost-effective and portable sensors to detect other metal ions and biomolecules in the future.