In this paper, a temperature and humidity chipless RFID sensor is presented. A conventional chipless RFID interrogation process is used to extract the tag identification as well as the two physical parameters. A model describing the evolution of the resonance frequency of a scatterer as a function of temperature and humidity is introduced. It allows the two quantities to be extracted independently of each other while each of them impacts the resonance frequency. On average, an error lower than 0.7 • C is obtained on the temperature and less than 1.3% on the humidity.