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In this work, we present a thorough study of the optical properties of the layered orthorhombic compound In4Se3. The dielectric function—real and imaginary parts, the complex refraction index, the reflectivity, the absorption coefficient, and the conductivity of In4Se3 were calculated with the inclusion of the spin-orbit interaction, using an ab initio FP-LAPW method based on DFT. Also, generalized ellipsometry was employed for more precise measurement of the anisotropic dielectric functions for polarization along crystal a, b, and c axes of orthorhombic absorbing In4Se3 single crystals cut approximately parallel to (100) at photon energies from 0.76 to 3.1 eV. Our experimental results show a good agreement with our calculations. We discuss the location and nature of the main optical peaks appearing in the spectra. The obtained optical functions display a rather anisotropic behavior, mainly in the infrared-visible region. Our results seem to be predictive to a high extension, given the scarce experimental information about its optical properties.