We present a fiber optic relative humidity sensor. The sensor was fabricated by coating a thin polyvinyl alcohol/CoCl2 film on a 200-?m core plastic-cladding fiber; the film was deposited after removing the cladding of the fiber. It was found that bending the fiber leads to drastic changes in the response behavior. The sensor was compared against a commercially available relative humidity sensor and was found to be sensitive to relative humidity ranging from 3 to 90%. The sensor response is usually very fast, but varies at different humidity levels and has a good dynamical range. The sensor response is repeatable and fully reversible, and should be useful for monitoring relative humidity in harsh industrial environments.