The fabrication and characterization of an optical fiber pH sensor based on Lossy Mode Resonances (LMRs) is presented. PAH/PAA polymeric thin-films fabricated onto side-polished D-shaped optical fibers are used as LMR supporting coatings. The thickness of PAH/PAA coatings can be modified as a function of the external medium pH. As a consequence of this variation, the effective refractive index of the structure will change, producing a shift of the LMR. The fabricated sensor has been used to measure pH from 4.0 to 5.0. This pH sensor showed a sensitivity of 101.3 nm per pH unit, which means a resolution of ~6×10-4 pH units by using a conventional communications Optical Spectrum Analyzer (OSA), which is an improvement over commercial pH sensors.