A method for testing the physical and optical properties of hydrogel thin films is reported based on using long-period fiber gratings. For humidity levels from 50% to 70% RH, a wavelength decrease of $11.0\mathrm{nm}$ is observed, with a sensitivity of $0.6\mathrm{nm}/\%\mathrm{RH}$ . For humidity levels from 70% to 98% RH, a wavelength increase of $6.3\mathrm{nm}$ is seen, with a sensitivity of $0.2\mathrm{nm}/\%\mathrm{RH}$ . For humidity levels greater than 60% RH, the transmission loss at resonance increases rapidly, showing a high degree of sensitivity of $0.5\mathrm{dB}/\%\mathrm{RH}$ . The blueshift of the wavelength was seen to be due to a thickening of the overlay, while the redshift of wavelength is due to the contribution of the drop in refractive index of the overlay. The results match well with what is expected theoretically.