Here we present a new structure to fabricate optical fiber sensors. This structure consists on an etched single mode optical fiber coated with a thin film of a semiconductor material by means of sputtering. The optical fiber was etched with hydrofluoric acid until it reaches 19 μm of diameter. Then it was attached to a U-holder and coated with the indium oxide nanocoating. The thin film generates a lossy mode resonance and when the relative humidity increases, the wavelength of the resonance shifts. Changes of 30% to 80% RH have been applied to the device obtaining a wavelength shift of 12 nm.