Soft material hydrogel sensors have seen increased interest recently. Most of these sensors are used in an aqueous environment. In this study, we depart from this trend and analyze the ability of a periodic hydrogel structure to respond to variations in ambient humidity through an optical change. First, a polyacrylamide inverse opal hydrogel structure was created from a colloidal crystal template. Next, this material was tested under various humidity conditions and responded to these changes by shifting its optical reflection peak noticeably within the visible wavelength range. This effect opens the doors for these materials as humidity sensors. The kinetics of the peak shifts was also observed, showing a rapid response to ambient humidity changes. Finally, the structural dimension change is compared through peak shifts, Fabry-Perot fringes of the optical cavity, and scanning electron microscopy observations.