Indium sulfide (In2S3) thin films were deposited on soda lime glass (SLG) substrate by radio frequency (RF) magnetron sputtering technique at 150 °C and then thermally annealed under argon (Ar) atmosphere at 350 °C and 450 °C for a period of 30 min. The effect of post-thermal annealing on the structural, morphological, and optical properties of the films were investigated. The formation of the stable tetragonal β-In2S3 was confirmed by X-ray diffraction (XRD) analysis. It was seen that the thermal annealing treatment at 450 °C improved the crystallization of the films. The change in the surface morphology of the films depending on the post-thermal annealing process were determined by atomic force microscopy (AFM) and scanning electron microscopy (SEM) analyses. The energy dispersive X-ray spectroscopy (EDX) analysis indicated that the films had slightly sulfur (S) deficit composition and the concentration of S slightly increased with the thermal annealing process. The room temperature (RT) photoluminescence (PL) spectra revealed that the films included sulfur vacancies (VS: donor), indium (In) vacancies (VIn: acceptor), indium interstitial (Ini: donor) and oxygen (O) in vacancy of sulfur (OVs: acceptor) defects with strong and broad emission bands at around 1.70 eV, 2.20 eV, and 2.71 eV.