In this study, a compact and flexible chipless radio frequency identification humidity sensor with dimensions of 21 × 21 mm2 is realized. The resonator structure consists of a square open ring and a U-shaped ring on a Rogers TMM 4(tm) substrate. The Kapton dielectric constant is very sensitive to changes in ambient humidity, and the sensor is capable of detecting humidity when the resonator is covered with a Kapton film. The amplitude of the radar scattering cross section increases monotonically from −65.66 to −58.14 dB with an offset of 7.52 dB when the relative humidity (RH) increases from 40 to 90%. The chipless sensor designed in this study has a compact size of only 21 × 21 mm2 with good resonance characteristics, and with the Kapton thin film covering, the sensor exhibits an excellent sensitivity of 0.15 dB/% RH. It also has a wide humidity measurement range from 40 to 90%.