Organic transistors having a layered structure are susceptible to interfacial problems such as fracture, delamination, and slip, which lead to poor stability of the device under mechanical stress. Here, we demonstrate highly stretchable, high-mobility, and free-standing coplanar-type all-organic transistors based on deformable solid-state elastomer electrolytes using ionic thermoplastic polyurethane (i-TPU), thereby showing high reliability under mechanical stimuli as well as low-voltage operation. Unlike conventional ionic dielectrics, the i-TPU electrolyte prepared herein has remarkable characteristics, i.e., a large specific capacitance of 5.5 μF/cm2, despite the low weight ratio (20 wt％) of the ionic liquid, high transparency, and even stretchability. These i-TPU-based organic transistors exhibit a mobility as high as 16.5 cm2V-1s-1, high bendability (Rc, radius of curvature: 7.2 mm), and good stretchability (60％ tensile strain). Moreover, they are suitable for low-voltage operation (VDS = ？1.5 V, VGS = ？2.5 V). In addition, the electrical characteristics such as mobility, on-current, and threshold voltage are maintained even in the concave and convex bending state (bending tensile strain of ~3.4％), respectively. Finally, we could fabricate free-standing, fully stretchable, and semi-transparent coplanar-type all-organic transistors by introducing a PEDOT:PSS layer as source/drain and gate electrodes, thus achieving low-voltage operation (VDS = ？ 1.5 V, VGS = ？ 2.5 V) and an even higher mobility of up to 17.8 cm2V-1s-1. Moreover, these devices withstand stretching up to 80％ tensile strain. We believe that our i-TPU-based device design will provide a rational guide to flexible and stretchable electronics.