The present study details a simple and cost-effective method of fabricating a thin film sensor for acid vapours. The sensor was prepared by the functionalization of chitosan (CS) with weakly emissive 8-hydroxy-2-methyl-quinoline-5,7-bis(N,N-dimethyl)sulfonamide (HQSA). Simple drop-casting followed by a dip-coating technique was adapted for the preparation of the CS–HQSA film. The hydrogen bond assisted functionalization was confirmed using FT-IR spectroscopy, and the morphological changes of CS after functionalization with HQSA were identified by SEM analysis. Furthermore, the prepared film was demonstrated to be an acid vapour sensor. The sensing was achieved through a reversible protonation of the nitrogen atom in the quinoline (HQSA) ring which induces a photoelectron transfer, causing significant colour change and a fluorescence spectral shift from 450 nm to 515 nm. The newly prepared thin film is sensitive towards acid vapours with a detection limit of 12 μM and a response time of 5 s. Also, the observed quantum yield (23%) and fluorescence lifetime values (6 ns) prove the efficiency of the CS–HQSA film. The above results prove that CS–HQSA would be an efficient chemosensor for acid vapours.