The detection of hydrogen sulfide (H₂S) is essential because of its toxicity and abundance in the environment. Hence, there is an urgent requisite to develop a highly sensitive and economical H₂S detection system. Herein, a zinc phthalocyanine (ZnPc) thin film-based K+-exchanged optical waveguide (OWG) gas sensor was developed for H₂S detection by using spin coating. The sensor showed excellent H₂S sensing performance at room temperature with a wide linear range (0.1 ppm – 500 ppm), reproducibility, stability, and a low detection limit of 0.1 ppm. The developed sensor showed a significant prospect in the development of cost-effective and highly sensitive H₂S gas sensors.