Scandium oxide (Sc2O3) films are deposited by reactive magnetron sputtering. The effects of annealing on the structure, as well as the optical, mechanical, and electrical properties are investigated. The detailed investigation of the mechanical properties and high load indentation resistance in combination with electrical and optical properties is done. It is noted that after annealing at 300C, the extinction coefficient and refractive index slightly decreases. It is reported that before and after annealing, Sc2O3 films with cubic phase remains very hard (19 GPa) and resistant to high load (up to 1N) indentation test. Moreover, the effects of post-deposition annealing on electrical characteristics of metal–insulator–semiconductor (MIS) structures with Sc2O3 gate dielectric layer are investigated. After the annealing procedure, the MIS structures are characterized by lower frequency dispersion, lower flat-band voltage value, and lower effective charge density. The insulating properties remain at similar level when compared with the devices fabricated with as-deposited dielectric material. The findings that are in this study make the investigated Sc2O3 films a promising material for various applications in optics, optoelectronic, and electronic semiconductor structures.