A novel dye incorporated into microporous material is introduced for optical humidity sensing. Methylene blue (MB) was encapsulated into the protonated mordenite zeolite (HMOR) via ion exchange reaction. The dye molecules are strongly retained in the channels of the zeolite and take part in the protonation/deprotonation reactions reversibly. The mechanism of the sensor is based on the protonation or deprotonation of the dye molecules which are associated with the desorption or adsorption of water molecules by the zeolite, respectively. The spectral changes due to different humidity levels are probed by diffuse reflectance spectroscopy. Discs prepared from 200mg of the dye-loaded zeolite provide a thickness of ≈0.8mm and show good characteristics for an optical humidity sensor. They showed a linear response range from 9 to 92% relative humidity (r² > 0.99), and good stability and reversibility. The sensor operates at either of the two 650 or 745nm bands but it exhibits a higher sensitivity for the measurements performed at 650nm. The sensor demonstrates relatively fast response and recovery times about 2min in the direction of adsorption and about 4min in the direction of desorption of water.