<p>Lead-free halide double perovskites have attracted considerable attention due to their promising optoelectronic and thermoelectric properties. In this work, density functional theory (DFT)-based simulations using WIEN2k code were performed to investigate the structural, optical, elastic, electronic, thermodynamic, thermoelectric, and photocatalytic properties of vacancy-ordered Na₂Sn(Cl/Br)₆ perovskites. The full-potential linearized augmented plane wave (FP-LAPW) method was employed to accurately evaluate their physical properties. Both compounds exhibit direct band gaps of 2.77/1.12&#xa0;eV (GGA), 3.63/2.40&#xa0;eV (GGA + SOC), and 3.98/3.28&#xa0;eV (hybrid HSE06), respectively, along with strong optical absorption of 64.3 × 10⁴&#xa0;cm⁻¹ and 60.3 × 10⁴&#xa0;cm⁻¹ in the visible/ultraviolet region of the light spectrum, making them promising candidates for photovoltaic and optoelectronic applications. Structural and thermodynamic stability is confirmed through tolerance factors of 0.90 and 0.89, negative formation energies of − 1.9174 and − 0.1673, respectively, phonon and ab initio molecular dynamic simulations, and thermodynamic assessments. Elastic and mechanical parameters reveal their ductile and anisotropic mechanical character. Moreover, high Seebeck coefficients of 1550/1580 in the <i>n</i>-type/<i>p</i>-type region for Na₂SnCl₆ and 973/765 in the <i>n</i>-type/<i>p</i>-type region for Na₂SnBr₆, notable electrical conductivity of 2.34 × 10¹⁹/2.12 × 10²⁰ in <i>n</i>-type/<i>p</i>-type region for Na₂SnCl₆ and 1.16 × 10²⁰/3.60 × 10²⁰ in <i>n</i>-type/<i>p</i>-type region for Na₂SnBr₆, and ZT (0.44 and 0.37) underscore their potential for thermoelectric device applications. Band-edge alignment under different exchange–correlation functionals, especially for the PBE-GGA functional, with water redox potentials further suggests their suitability for solar-driven hydrogen evolution, positioning them as multifunctional materials for sustainable energy technologies.</p>