<p>We report a comprehensive first-principles study of the structural, electronic, magnetic, optical, and thermoelectric properties of the double perovskite K₂OsCl₆ using density functional theory (DFT) within the generalized gradient approximation (GGA). The calculations predict a ferromagnetic ground state with a total magnetic moment of ~ 1.99 µB per formula unit, largely arising from Os atoms (~ 1.33 µB). Spin-resolved electronic band structures reveal robust half-metallicity, featuring a metallic majority-spin channel and a direct minority-spin band gap of ~ 0.8&#xa0;eV, yielding nearly 100% spin polarization at the Fermi level. The optimized lattice constant of 10.45 Å shows excellent agreement with experimental data. Optical analysis demonstrates strong spin-dependent behavior in the visible and ultraviolet regions, with a peak absorption coefficient of ~ 1.5 × 10⁵ cm⁻¹ at 6&#xa0;eV and static dielectric constants of 4.2 (spin-up) and 3.8 (spin-down). The refractive index exhibits anomalous dispersion, reaching refractive index values up to 2.8 in the visible range. Thermoelectric calculations indicate a Seebeck coefficient of ~ 180 µV/K at 900&#xa0;K, electrical conductivity of 4.5 × 10⁵ Ω⁻¹ m⁻¹, and low thermal conductivity of 2.8&#xa0;W m⁻¹ K⁻¹ at room temperature.</p>