Two-dimensional (2D) β-In2S3 with a natural defective structure is a promising semiconductor for electronic and optoelectronic applications. Herein, oxidation of 2D β-In2S3 is investigated by photoluminescence, Raman scattering, and X-ray photoelectron spectroscopy studies for understanding its optoelectronic properties. Our results show that the intrinsic structural defects of sulfur vacancies induce the oxidation of β-In2S3, which can act as active sites to adsorb oxygen in air. Oxygen atoms react with indium atoms to form an inner layer of In2S3-3xO3x and outer layer of In2O3, resulting in intriguing optical properties over the exposure time to air. Moreover, the energy level diagram based on the defect-mediated oxidation process is presented. Additionally, the effect of oxidation under ambient air on β-In2S3 based photodetector and field effect transistor is investigated. Our study provides an in-depth understanding of the oxidation process of 2D β-In2S3 and paves a fundamental step for its potential applications in future electronics and optoelectronics. Display omitted •Defects can act as active sites for molecule adsorption in air to induce oxidation.•β-In2S3 is a typical natural defective crystal.•Photoluminescence of two-dimensional β-In2S3 varies as the exposure time to air.•Performance of photoresponsivity degrades while mobility increases after oxidation.