The design of effective energy systems is crucial for the development of flexible and wearable electronics. Regarding the direct conversion of heat into electricity, thermoelectrochemical cells (TECs) are particularly suitable for low-grade heat harvesting to enable flexible and wearable applications, despite the fact that the electrolyte leakage and complex packaging issues of conventional liquid-based TECs await to be further addressed. Herein, a quasi-solid-state TEC is assembled using the polyacrylamide/acidified-single-walled carbon nanotube (PAAm/a-SWCNT) composite hydrogel, developed via a facile in situ free-radical polymerization route with tin(iv) chloride/tin(ii) chloride (Sn4+/Sn2+) as the redox couple. The as-fabricated TEC with a 0.6 wt% a-SWCNT content presents a large thermoelectrochemical Seebeck coefficient of 1.59 ± 0.07 mV K-1 and exhibits excellent stability in thermoelectrochemical performance against large mechanical stretching and deformation. Owing to this superior stretchability, the as-fabricated TEC is further assembled into an energy-autonomous strain sensor, which shows high sensitivity. The strategy of utilizing a quasi-solid-state TEC for energy-autonomous strain sensing unveils the great potential of heat-to-electricity conversion in flexible and wearable electronics.