The present work aims to use the Quantum Espresso software to theoretically investigate the spontaneous polarization and refractive indices of barium titanate (BaTiO3) in its tetragonal phase, and their dependence on the considered DFT exchange-correlation functionals. Among the functionals implemented in Quantum Espresso, twenty-four functionals were considered. The calculations, in which pseudopotentials of type ultrasoft were used, show that the values of all studied magnitudes depend on the used exchange-correlation functional. According to the obtained results, for the considered functionals, it appears that there is a linearity between the spontaneous polarization and the ionic shift difference Δz(Ti)-Δz(Oz); this linearity can be explained by the fact that the dipolar interaction Ti-Oz is the main contributor to the spontaneous polarization, being seen that: (i) the spontaneous polarization is a consequence of the coupling between the displacements of the constituent ions and their effective charges, (ii) the ionic shifts lead to a decrease in the Ti-Oz distance and an increase in the Ti-Ox and Ti-Oy distances, and then an increase in the Ti-Oz dipolar interaction compared to the Ti-Ox and Ti-Oy dipolar interactions, and (iii) the Ti and Oz ions possess the highest Born effective charges (in absolute values), in the polar direction, compared to the other ions, as reported in the literature. However, the closest values of the spontaneous polarization to the experimental data were obtained for the exchange-correlation functionals VDW-DF-C09, PZ, VDW-DF-CX, VDW-DF3-OPT1, VDW-DF2-C09, WC, and PBESOL. For these functionals, the refractive indices of ordinary and extraordinary polarizations were studied as a function of the light wavelength, in the range 160–940 nm. The obtained results show that the behavior of both refractive indices with respect to the wavelength, for visible light, is quit similar to that of Sellmeier equation with good agreement with the experimental data, particularly for the van der Waals functionals VDW-DF3-OPT1, VDW-DF-CX, VDW-DF2-C09, and VDW-DF-C09.