In the present study, we have used density functional theory (DFT) with the generalized gradient approximation (GGA), as implemented in the Quantum ESPRESSO code, to investigate the structural, electronic, and optical properties of the half-Heusler alloy LiGaSi. All the calculations were performed using the thermo_pw code, which is a post-processing tool for Quantum ESPRESSO. We applied 3% compressive and tensile strains on the material in our analysis. The indirect band gap of LiGaSi increases under tensile strain and decreases under compressive strain. In addition, the results of the optical characteristics showed that the dielectric constant of the LiGaSi alloy increases with the tensile strain and slightly decreases with the compressive strain. Additionally, for the tensile strain of 3%, the absorption coefficient in the UV area is very high, at around 204 × 104 cm−1, while attenuation decreases in the UV region, indicating a less scattering effect. Such results suggest that said material is a promising candidate for the optoelectronic applications due to its tunable electronic and optical properties.