A lossy mode resonance (LMR)-based fiber optic humidity sensor coated with polymeric humidity-sensitive materials is proposed. Titanium dioxide (TiO₂) is deposited on the multimode fiber core using the layer-by-layer assembled (LBL) technique to excite LMR effects. The refractive index sensitivity of the LMR sensor is systematically investigated through both theoretical analysis and experimental validation. Two distinct humidity sensors are fabricated by employing polyvinyl alcohol (PVA) and polyimide (PI) as the functional sensing layers, respectively, where humidity detection is achieved by monitoring the resonance wavelength shift in the absorption spectrum. Experimental results demonstrate that both sensors exhibit effective humidity-sensing capabilities with comparable performance characteristics. Notably, the PVA-based sensor achieves a maximum sensitivity of 2.94 nm/%RH (relative humidity) within the 60% RH—90% RH range. Owing to their high sensitivity, excellent reversibility, and outstanding repeatability, the proposed sensors exhibit significant potential for practical applications in humidity monitoring.