The design of ultrasensitive ionic thermopiles is important for low-grade heat collection and temperature sensing. However, high-quality ionic thermoelectric materials with negative thermopower have been rarely reported to date. Effective adjustment of the interaction between the polymer network and the electrolyte anion/cation is an important method to achieve notable thermopower. Here, we demonstrate an ionic hydrogel thermoelectric material with giant negative thermopower obtained by synergistic coordination and hydration interactions. The ionic hydrogel, made of polyvinyl alcohol and sodium hydroxide, is prepared by simple dry-annealed process and exhibits a thermopower of up to −37.61 millivolts per kelvin, an extremely high absolute thermopower for electronic and ionic conductors. This ionic hydrogel is promising for the design of high-thermopower ionic thermoelectric materials and the low-grade heat energy harvesting.