<p>Harvesting low-grade heat is a sustainable way to power wearable electronics, and quasi-solid-state ionic thermoelectric cells offer a flexible, low-cost option. Their use, however, has been limited by a key trade-off: miniaturization reduces the internal thermal gradient and compromises performance. Here, we address this challenge with an ultrathin asymmetric architecture that separates thermal energy harvesting from the conventional reliance on a sustained through-plane temperature gradient. The design couples thermally driven ionic modulation at one interface with engineered pseudocapacitive charge storage at the other. Our 1-mm-thick device delivers an open-circuit voltage of 0.1 V, a power density of 1.6 W m⁻², and an energy density of 1500 J m⁻² using near-body heat. An array of 20 cells generates 1.9 V and a peak power of 23 W m⁻², enabling continuous smartwatch operation. This strategy provides a practical route to ultrathin ionic thermoelectric cells for self-powered wearable systems.</p>