This paper describes a wireless ammonia sensor that employs the resonance frequency change in a curved magnetic ribbon coated with poly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonate) (PEDOT/PSS). As PEDOT/PSS expands due to the sorption of ammonia molecules, the curvature of the ribbon is decreased with increasing ammonia concentration, P_am. The decrease in curvature reduces the mechanical resonance frequency from 105 kHz at P_am = 0% to 89 kHz at P_a= 10%. We also confirmed that the resonance frequency is proportional to the curvature and that the proportional constant is 0.33 kHz/m^-1. Since the mechanical vibration of the ribbon is excited by applying AC and DC fields and is detected by monitoring the voltage induced on a pickup coil, the P_am value around the ribbon can be detected wirelessly.