Real-time and continuous wireless measurement of human body temperature could enable a better control of many pathologies such as the wounds infection after surgery and the evolution of epidemics involving fever rush, as well as the monitor of athletic activities. This paper describes an RFID passive UHF epidermal sensor suitable to be directly attached onto the human skin by means of a bio-compatible transpiring Poli(ε-caprolacton) (PCL) membrane. The antenna elements provide a broad matching band and even a post-fabrication tuning mechanism to better manage the specific placement over the body. The temperature is directly measured by the EM4325 microchip, also providing RFID communication capabilities. The epidermal sensor, that can be read up to 35 cm in case of 0.5 W EIRP emitted by the reader, has been moreover thermally calibrated versus a thermocouple and then applied to the measurement of human body temperature in both static and dynamic conditions with an accuracy of about 0.25°C with respect to reference measurements.