A novel pH sensor based on the deposition of electrostatic self-assembled polyallylamine hydrochloride and polyacrylic acid on the cladding of a long-period fiber grating has been designed. The overlay thickness is of the order of magnitude of the light wavelength used to interrogate the sensor. A theoretical model of multilayer cylindrical waveguides based on coupled-mode theory has been used to predict the position of the attenuation bands as a function of the overlay thickness. A study of the role that pH plays in the thickness of the sensitive overlay was carried out. Experimentally, a wavelength shift of the attenuation bands of up to 85 nm was obtained in the 4-7 range of pH units. The results are reproducible, which indicates the possibilities of a multiple-use sensor.