The present invention relates to a device (10) and method for non-invasively measuring a hydration state of a living being (62). To improve with respect to the convenience-to-use, the cost-effectiveness and the accuracy of the delivered results, the proposed device comprises a first light source (14) for emitting light of a first wavelength into a tissue portion (12) of the living being (62), polarization means (18) for polarizing at least part of the emitted light prior to an interaction of the emitted light with the tissue portion (12), a first light detector (26) for detecting polarized light of the first wavelength reflected at the tissue portion (12) and processing means (32) configured to derive a tissue hydration parameter from the detected reflected polarized light and to determine a body hydration index based on said tissue hydration parameter.