The ever-increasing demand for improved quality of daily life drives the rapid evolution of wearable electronics to monitor personal biosignals. Body temperature is one of the critical factors for the diagnosis of health condition. For accurate diagnosis of health conditions, temperature sensor must be able to detect minute changes that may be as small as 0.1 ℃ and stably operate under mechanical deformations during body movements. Here, we demonstrate a deformation-insensitive, flexible, and highly sensitive temperature sensor based on polymer composite with semi-crystalline polymer microdomains. Unlike previously reported resistive temperature sensors, our sensor shows an extremely large change in resistance due to the volume expansion of the semi-crystalline polymer microdomains, as well as the TCR of graphene. The optimized temperature sensor exhibits an ultrahigh sensitivity of 6.94% ℃-1, high sensing accuracy of 0.1 ℃, and stable operation under mechanical deformation.