This paper proposes for the first time, a group delay based humidity sensor for chipless RFID applications. The sensor network consists of cascaded group of microstrip transmission line sections coupled at alternative ends. Silicon nanowires are deposited on the transmission line sections. The nanowires are extremely sensitive to humidity variations, which make it a good candidate for sensor applications. The sensor shows a very good sensitivity to humidity in phase and group delay at a bandwidth of 750 MHz from the fundamental frequency and a very good reversibility is also achieved. The variation of phase and group delay in accordance with the humidity absorption is explained for a single group of C-sections. A proof of concept is developed for the prototype and is validated using measurement results. Moreover, measurement for a temperature variation of ±10°C was also performed and no significant variation was observed. The obtained results confirm the robustness of these kinds of tags in humidity sensor application.