Abstract: In this paper, a chipless radio frequency identification (RFID) humidity sensor based on U-shaped resonant units is researched, designed and implemented, which is combined with a Vivaldi wide-band antenna to construct a passive and wireless humidity sensing system, which can realize the real-time monitoring of environmental humidity. The chipless RFID humidity sensor design scheme consists of eight U-shaped resonant units and a coupled microstrip line. In this design, the symmetrical resonant units on both sides are used to calibrate the effect of ambient temperature on the sensor, and one of the units is covered by a polyvinyl alcohol (PVA) moisture-sensitive material. The six U-shaped units in the center are used to encode the ID information of the RFID humidity sensor. In addition, this work optimizes a wide-band Vivaldi antenna using a rectangular ring slotting technique and combines it with the sensor to achieve wireless passive humidity sensing. Through wired and wireless experiments, the sensor demonstrates high humidity detection sensitivity. The wired experimental data show that a frequency offset of 118 MHz is generated over a relative humidity (RH) range of 57% RH to 71% RH, with an average sensitivity of 8.4 MHz/% RH. The wireless experimental results show that a frequency offset of 103 MHz is generated over a relative humidity range of 49% RH to 70% RH, with an average sensitivity of 4.9 MHz/% RH. The experimental results show that the passive wireless humidity sensing device has high sensitivity and can provide an important solution for wireless passive humidity detection.