Photocatalytic technology has been hailed as one of the most promising ways to alleviate and potentially avoid both the global energy environment and consumption issues. In this regard, the perovskite RGaO3 (R = Rb and Na) oxides are investigated for water splitting applications. DFT based CASTEP code is utilized by employing GGA-PBE exchange-correlation functional. According to structural analysis, compounds have 221 space groups (cubic) with 5 atoms. The structural analysis and band structures of NaGaO3 and RbGaO3 crystals were described as having an indirect bandgap with 3.11 eV and 2.15 eV energy, respectively. Absorption, reflectivity, loss function, conductivity, refractive index, and dielectric function were all explored in terms of optical characteristics. According to the mechanical properties, compounds RGaO3 (R = Rb and Na) are mechanically stable, ductile, anisotropic, and hard by Born stability. These materials are best suited for photocatalytic applications to produce H2 fuel with the highest efficiency.