<p>Halide double perovskites Tl₂NaSbI₆ and Tl₂NaBiI₆were investigated using density functional theory to explore their structural stability, electronic structure, optical response, photovoltaic performance, and photocatalytic activity. The thermodynamic, mechanical and dynamical stability was confirmed by the tolerance factor analysis, negative formation and decomposition energies, mechanically stable elastic constants, and the absence of imaginary phonon frequencies. Tl₂NaSbI₆ has higher elastic constants and bulk modulus while Tl₂NaBiI₆ is comparatively more ductile. The electronic band structure calculations show that both compounds are indirect bandgap semiconductors with band gaps of 1.51–1.88&#xa0;eV for Tl₂NaSbI₆ and 2.10–2.32&#xa0;eV for Tl₂NaBiI₆. The band-edge states are strongly affected by spin–orbit coupling, which is caused by the presence of heavy elements. The observed electronic and optical behaviour is explained by strong hybridization between I-5p, Tl-p and Sb/Bi-p orbitals, as revealed by density of states analysis. The optical properties show high visible to ultraviolet absorption, high dielectric response and good optical conductivity, Tl₂NaSbI₆ shows higher optical activity. The effective mass calculations indicate that the charge carriers in Tl₂NaBiI₆ are comparatively lighter, and the estimated exciton binding energies of 0.218&#xa0;eV and 0.25&#xa0;eV for Tl₂NaSbI₆ and Tl₂NaBiI₆, respectively, indicate efficient exciton dissociation and charge separation. Band-edge alignment analysis also shows that Tl₂NaSbI₆ meets the energetic requirements for overall water splitting, whereas Tl₂NaBiI₆ is favourable for oxidation-driven photocatalytic processes. The results show that the substitution of Sb for Bi is an effective way to tune the optical, electronic and energy conversion properties of Tl-based halide double perovskites.</p>