The fabrication and characterization of a planar silicon-based sensor for free chlorine, present in the form of hypochlorous acid, is presented. The sensor uses a three-electrode electrochemical cell, with thin-film platinum working and counter electrodes, and a thin-film partially chloridized silver reference electrode. The working electrode is covered with a photopolymerized polyHEMA hydrogel membrane, which serves as a diffusion-limiting membrane. All electrodes are contacted by polysilicon films isolated with silicon nitride, to obtain a precise definition of the electrode area and to make the encapsulation less critical. The optimal polarization potential for hypochlorous acid detection at which there is no interference from the reduction of dissolved oxygen is +350 mV versus SCE. Calibration curves show an excellent linearity (R2 = 0.9996, n = 21) and a sensitivity of approximately 10 nA/(mg chlorine/1). The detection limit (S/N = 3) is approximately 1 μg chlorine/1 (1 ppb).