The electronic structure, optical response, and vibrational properties of the In_0.3Ga_0.7Se crystal were investigated using first-principles density functional theory. The dielectric tensor components were calculated, enabling a detailed analysis of the refractive index, extinction coefficient, absorption coefficient, optical conductivity, and reflectivity for light polarized parallel and perpendicular to the optical axis. The results identify In_0.3Ga_0.7Se as a direct-gap semiconductor with a fundamental band gap of 1.91 eV, highlighting its potential for optoelectronic applications in the visible spectral range. To elucidate the electronic structure, atom-projected partial densities of states were analyzed, revealing the contributions of In, Ga, and Se orbitals to the valence and conduction bands. The vibrational properties were examined through a combined theoretical and experimental approach using density-functional perturbation theory and Raman spectroscopy. A direct comparison between the calculated phonon modes and the experimental Raman spectrum allowed the assignment of five Raman-active modes at 40, 111, 180, 225, and 256 cm^-1, corresponding to A₁, B₁, E₂, and E₁ symmetry representations. The associated atomic displacement patterns were analyzed to clarify the nature of lattice vibrations in the mixed-cation system. In addition, the temperature dependence of the heat capacity was calculated over the range of 0–400 K, providing insight into the thermodynamic behavior of In_0.3Ga_0.7Se. The overall agreement between the calculated phonon frequencies and experimental Raman data confirms the reliability of the first-principles approach and establishes a consistent description of the electronic, optical, vibrational, and thermodynamic properties of this non-equiatomic In–Ga–Se solid solution