AbstractIn the quest for nontoxic and stable perovskites for solar cells, we have conducted a systematic study of the effect of chalcogen content in oxychalcogenide perovskite by using DFT and quasi‐particle perturbation theory. We explored the changes in the electronic structure due to the substitution of O atoms in NaNbO₃ and NaTaO₃ perovskite structures with various chalcogens (S, Se, Te) at different concentrations. Interestingly, the introduction of the chalcogen atoms resulted in a drastic reduction in the electronic band gap, which made some of the compounds fall within the visible range of the solar spectrum. In addition, our analysis of the electronic structure shows that the optical transition becomes direct as a result of the strong hybridization between the orbitals of the transition metal and those of the chalcogen ion, in contrast to the indirect band feature of NaNbO₃ and NaTaO₃. We identified candidates with a high theoretical solar conversion efficiency that approached the Shockley–Queisser limit, which makes them suitable for thin‐film solar cell applications. The present work serves as a guideline for experimental efforts by identifying the chalcogen content that should be targeted during the synthetic route of thermodynamically stable and strongly photoactive absorbers for oxychalcogenide perovskites in thin‐film solar cells.