In this theoretical study, we examined the structural, electronic, optical, elastic, and thermoelectric properties of Cs2NaBiX6 (X = Cl, Br, I) halide double perovskites. The calculated formation energies are negative, affirming the stability of these compounds and their suitability for experimental synthesis. Our investigation has revealed the presence of strong covalent bonds within the [Bi/NaX6] clusters. The electronic properties demonstrate the indirect bandgap of 1.94, 3.15, and 3.24 eV for Cs2NaBiI6, Cs2NaBiBr6, and Cs2NaBiCl6, respectively, calculated using modified Becke-Johnson potential plus spin–orbit coupling (mBJ + SOC) approximation. In the visible spectrum, Cs2NaBiI6, Cs2NaBiBr6, and Cs2NaBiCl6 exhibit average rate transmittance of 53 %, 83 %, and 84 %, respectively. The absorption spectrum, extending from visible to ultraviolet (UV) wavelengths, reaches 25 × 104/cm in the case of the iodine-based perovskite. The thermoelectric characteristics, involving Seebeck coefficient, electrical conductivity, and power factor, demonstrate the use of these perovskites as a suitable for solar cells and solar thermal energy conversion technologies.