In this study, ab initio density functional theory (DFT) calculations were conducted to comprehensively investigate the structural, mechanical, electronic, and optical properties of the double perovskite oxides Sr2PAlO6 and Sr2PGaO6. The structural analysis, supported by the Goldschmidt tolerance factor, confirms that both compounds adopt a stable perovskite structure. Furthermore, their calculated negative formation energies (−3.059 eV for Sr2PAlO6 and −3.367 eV for Sr2PGaO6) strongly indicate excellent thermodynamic stability, making them promising candidates for practical applications. Mechanical stability is verified through elastic constant calculations, which fully satisfy the Born and Huang mechanical stability criteria for cubic systems. The absence of any imaginary frequencies in the phonon dispersion spectra further confirms the dynamic stability of both structures. In terms of mechanical behavior, these materials demonstrate anisotropic and brittle characteristics, as evidenced by anisotropy factors deviating from unity and Poisson’s ratio values below 0.26. Electronic structure calculations reveal that both Sr2PAlO6 and Sr2PGaO6 exhibit semiconducting behavior with direct band gaps of 2.860 eV and 3.027 eV, respectively. Such values suggest they are suitable for use in optoelectronic devices operating in the visible to ultraviolet range. The optical properties, derived from the complex dielectric function, show strong and broad absorption features across the energy range of 0 to 12 eV, indicating their potential for efficient light-harvesting and energy conversion applications. In conclusion, the combined structural, mechanical, electronic, and optical analyses highlight that Sr2BPO6 (B = Al, Ga) double perovskites are promising multifunctional materials with significant potential for deployment in future optoelectronic and photovoltaic technologies.