Stretchable electronic materials and devices have important applications in flexible electronic systems including wearable electronics and bioelectronics. Convenient electricity generation such as thermoelectric conversion is required for the flexible electronic systems. Hence, it is development of high‐performance thermoelectric materials with high mechanical stretchability would be highly desirable. Here, stretchable and transparent ionogels with high thermoelectric properties are demonstrated. The ionogels made of elastomeric waterborne polyurethane and 1‐ethyl‐3‐methylimidazolium dicyanamide (EMIM:DCA, an ionic liquid) are prepared by solution processing. Their mechanical and electrical properties depend on the loading of EMIM:DCA. The ionogels with 40 wt% EMIM:DCA can have a high mechanical stretchability of up to 156%, low tensile strength of 0.6 MPa, and low Young's modulus of 0.6 MPa. They also exhibit a high ionic thermovoltage of 34.5 mV K−1, high ionic conductivity of 8.4 mS cm−1 and low thermal conductivity of 0.23 W m−1 K−1 at a relative humidity of 90%. As a result, it can have a high ionic figure of merit (ZTi) of 1.3 ± 0.2. Both the thermovoltage and the ZTi value are the highest for stretchable thermoelectric materials. They can be used in ionic thermoelectric capacitors to convert heat into electricity. Ionogels of WPU and 1‐ethyl‐3‐methylimidazolium dicyanamide (EMIM:DCA, an ionic liquid) have high thermoelectric performance, high mechanical stretchable, and high optical transparency. They have a high mechanical stretchability of up to 156% and high ionic thermovoltage of 34.5 mV K−1. Their ionic figure of merit (ZTi) can be up to 1.3 ± 0.2.