Abstract <p>The current study broadly explores the structural, electronic, optical, mechanical, thermodynamic, thermoelectric, and photocatalytic water-splitting performance of halide double perovskites K₂RbGaX₆ (X = Cl, Br), using density functional theory (DFT). The structural and dynamic stability is confirmed through tolerance factor, formation energy, and phonon dispersion analysis with no negative (imaginary) frequency vibrations. Moreover, ab initio molecular dynamic simulation confirms the thermal stability at 300 K with no structural deformations. Electronic properties are investigated using different exchange-correlation potentials to get the accurate band gaps of the considered materials, indicating that both the studied materials exhibit direct band gap semiconducting nature, spanning from 2.15 eV to 3.33 eV, respectively. Optical properties reveal that both the studied materials are ultraviolet absorbers with a significant redshift in the absorption edge due to variation in their band gaps. Thermoelectric properties analysis reveals that both the studied materials have a high Seebeck coefficient of 0.00152 V/K and 0.00128 V/K, a high-power factor, low thermal conductivity, and favorable figure of merit (ZT) having the values of 0.74 and 0.53 for K₂RbGaX₆ (X = Cl, Br), respectively. Additionally, the photocatalytic water-splitting behavior of both the studied materials reveals their significance in hydrogen production through photocatalysis. The overall investigations suggest that both the studied materials have multifunctional potential in optoelectronics and photocatalysis, encouraging a noteworthy step onward in the design of next-generation materials for renewable energy technologies. These findings open new avenues for the experimental synthesis and application of these compounds in integrated energy conversion systems.</p> Graphical Abstract <p></p>