A novel ammonia gas sensor using a polyelectrolyte thin film composed of poly(diallyldimethylammonium chloride) (PDDA) and poly(acrylic acid) (PAA), PDDA/PAA, is demonstrated with a long-period grating (LPG) optical fiber. PAA could act as a selective receptor for ammonia binding, which induced significant changes in the optical thickness and density of the film owing to the pronounced electrostatic interaction between the alternate layers.