Abstract <p>Halide double perovskites are promising lead‑free alternatives for optoelectronic and thermoelectric devices. Herein, we employ first-principles calculations based on the full-potential linearized augmented plane wave method within density functional theory to investigate the structural, electronic, optical, and thermoelectric properties of novel A₃AsI₆ (A = K, Rb, Cs) perovskites. The thermodynamic stability of these compounds is confirmed by negative formation energies and positive cohesive energies, while their structural stability is supported by tolerance factors (τ_G = 0.9, 0.9, and 1.0). Using PBEsol-GGA for structural optimization and the TB-mBJ approximation for electronic and optical analyses, we find that K₃AsI₆ exhibits an indirect bandgap of 2.83 eV, whereas Rb₃AsI₆ and Cs₃AsI₆ display direct bandgaps of 2.84 eV and 2.86 eV, respectively. To overcome the limitation of PBEsol-GGA and TB-mBJ potential, we utilized HSE06 which improved the bandgap values of (2.9, 3.1, and 3.2 eV) for A₃AsI₆ (A = K, Rb, Cs). Optical studies reveal strong absorption in the ultraviolet region, with K₃AsI₆ showing a peak absorption coefficient of 1.32 × 10⁶ cm^-1 at 9.8 eV, indicating its potential for optoelectronic devices. Thermoelectric analysis over 0–800 K demonstrates <i>p</i>-type behavior for Rb₃AsI₆ and Cs₃AsI₆, with Cs₃AsI₆ achieving a high figure of merit (ZT) of 0.78 at 500 K, highlighting its suitability for thermoelectric applications. The band edge positions also align favorably for photocatalytic water splitting. These findings establish A₃AsI₆ perovskites as promising candidates for next-generation optoelectronic, energy-harvesting, and photocatalytic technologies.</p> Impact statement <p>This work unveils the first comprehensive theoretical investigation of the unexplored A₃AsI₆ (A = K, Rb, Cs) halide double perovskite family, establishing them as lead-free multifunctional materials with exceptional promise for next-generation optoelectronic, energy-harvesting, and photocatalytic technologies. By combining advanced first-principles calculations with the accurate TB-mBJ potential, we demonstrate that Rb₃AsI₆ and Cs₃AsI₆ exhibit direct bandgaps ideally suited for visible-light applications, while all three compounds display remarkably strong ultraviolet absorption (10⁶ cm^-1) surpassing many known perovskites. Notably, Cs₃AsI₆ achieves a high thermoelectric figure of merit (ZT ≈ 0.79 at 500 K), rivaling established thermoelectric materials, and the favorable band alignment with water redox potentials unlocks photocatalytic water-splitting capability. These findings not only fill a critical knowledge gap by introducing a completely novel perovskite system, but also establish design principles for discovering multifunctional halide double perovskites, potentially accelerating the development of sustainable energy-conversion devices.</p> Graphical abstract <p></p>