A tissue hydration monitor and method includes a sensor module having a plurality of LEDs positioned to emit a plurality of different wavelengths of light toward the user's skin and a detector that detects light transmitted and reflected through the user's skin to generate signals corresponding to an intensity of detected light at each of the different wavelengths. A processor/controller module generates a baseline hydration level based on the received signals, calculates a relative hydration level, and generates an output indicative of relative hydration personalized to the user. The housing is secured against the user's skin by an adhesive patch or a strap.