Abstract: This invention relates to a soft, battery-free, flexible, non-invasive, reusable hydration sensor adherable to even small-areas and curvilinear surfaces of a body. The hydration sensor measures volumetric water content in skin as a function of depth, and wirelessly transmits data to a portable smart device. The hydration sensor includes a top layer for thermal, chemical and mechanical isolation of the hydration sensor from an environment; a bottom layer operably placed on a target area of interest on the skin; and a flexible printed circuit board (f-PCB) disposed between the top layer and the bottom layer. The f-PCB contains electronics for sensing and wireless communication. The bottom layer operably serves as a direct interface between the f-PCB and the skin and comprises a flexible adhesive for attaching the hydration sensor to the skin.