In this paper, by using density functional theory (DFT), the structural, electronic, and optical properties of cubic and orthorhombic perovskites BaZrS3−xSex are explored. The lattice parameters increase when the substitution of S/Se atoms is applied, whereas the bulk moduli decrease. The density of states curves showed a strong hybridization between Zr-d and S-p/Se-p orbitals. Furthermore, the obtained bandgaps of the orthorhombic phases are direct (Γ–Γ), whereas they are indirect (R–Γ) for the cubic phase. When substituting sulfur with selenium, the bandgap decreases from 0.963 to 0.705 eV for the orthorhombic phase and from 0.655 to 0.288 eV for the cubic phase. The dielectric function showed that the optical bandgaps are between 0.7 and 0.5 eV for orthorhombic BaZrS3–xSex, which is in the infra-red spectrum, and it was between 0.7 and 0.25 eV for the cubic phase. The reflectivity was in the range of [26%–31%] at low energies, which can be considered quite reflective. The refractive index for the orthorhombic phase increased from 3.0 to 3.5 at low energies and from 3.6 to 3.9 for the cubic phase, which indicates high absorption at those values of energy. The result obtained indicates that chalcogenide perovskites BaZrS3−xSex are good candidates for future photovoltaic applications such as tandem solar cells.