Indium selenide (InSe), a member of chalcogenide semiconductors, has attracted immense attention due to its wide range of technological applications in solar cells, data storage, switching devices and diodes. In the present study, InSe thin films are deposited using thermal evaporation method, and post-annealing treatment has been performed in vacuum at different temperatures (150 °C, 200 °C, and ... [Show full abstract] 250 °C) to induce changes in structural, morphological and optical properties. As-deposited InSe thin film has monoclinic phase, and the crystallinity is found to increase with annealing at 150 °C and 200 °C. The phase transition from crystalline to amorphous phase is achieved with annealing at higher temperature (250 °C). The drastic change in the morphology with annealing temperature is clearly visible in field emission scanning electron microscope (FE-SEM) images. With annealing, the average transmission in the wavelength range of 900–2400 nm increases, and also the value of optical band gap increases from 1.12 to 1.42 eV. The observed change in the transmission and optical band gap is due to change in density of localized and/or delocalized defect states in the forbidden gap with post-annealing treatment. These results show that the post-annealing treatment has significant impact on structural, morphological and optical properties of InSe thin films.