The influences of annealing temperature (473–573 K) on the crystal structure, linear/nonlinear optical parameters, and electrical characteristics of 80 nm thick indium-doped tin oxide (ITO) thin films are investigated. Thermal annealing induces the crystal structure in the ITO. As-prepared and annealed ITO have various morphologies depending on the annealing temperature, such as nanoplates and dendritic and spherical nanoparticles. As the substrate temperature increased up to 370 K, the electrical resistivity and sheet resistance of as-prepared ITO decreased dramatically and then slightly decreased as the substrate temperature further increased. The electrical conductivity and activation energy for the various processes were estimated. The reflectance (R) and transmittance (T) data are used to calculate the linear/nonlinear optical constants and parameters. The optical bandgap increased from 3.18 to 3.8 eV as the annealing temperature increased from room temperature to 573 K. Crystallinity is improved due to the annealing and hence an enhancement in the optical energy bandgap is achieved. Meanwhile, high-temperature annealing reversibly affected the optical bandgap energy of ITO thin films via reduction and oxidation reactions. Thermal annealing of ITO films improves crystal structure, visual transparency, and electrical conductivity, making it the preferred material for optoelectronic devices and solar cells.