<jats:title>Abstract</jats:title><jats:p>Ionic thermoelectric materials can generate large thermal voltages under temperature gradients while also being low‐cost and environmentally friendly. Many electrolytes with large Seebeck coefficients are reported in recent years, however, the mechanism of the thermal voltage is remained elusive. In this work, three types of polyelectrolytes are studied with different cations and identified a significant contribution to their thermal voltage originating from a concentration gradient. This conclusion is based on studies of the loss and gain of water upon temperature changes, variations in conductivity with water content and temperature, and the voltages induced by changes in water content. The results are analyzed by the “hopping mode” dynamics of charge transport in electrolytes. The hydration of different cations influences the water concentration gradient, which affects the barrier height and ion‐induced potential in the electrodes. This work shows that the hydro‐voltage in ionic thermoelectric devices can be one order of magnitude larger than the contribution from thermodiffusion‐induced potentials, and becomes the main contributor to energy harvesting when implemented into ionic thermoelectric supercapacitors. Together with the rationalized theoretical discussion, this work clarifies the mechanism of thermal voltages in electrolytes and provides a new path for the development of ionic thermoelectric materials.</jats:p>< Réduire