<p>Addressing the demand for eco-friendly multifunctional materials, this study explores the pressure-responsive versatility of the K-based halide double perovskite K₂TlYCl₆ through a comprehensive theoretical investigation of its optical, electronic, thermoelectric, and photocatalytic properties under hydrostatic pressure (0–15 GPa). Using first-principles calculations, we confirm the structural and thermodynamic stability of K₂TlYCl₆ across the entire pressure range. Analysis of tolerance factor, octahedral factor, formation and cohesive energies, and phonon dispersion reveal that the compound retains its cubic crystal structure without undergoing any phase transition, highlighting its robust structural integrity under compression. Notably, a pressure-induced mechanical metamorphosis is observed: the compound exhibits ductility up to 10 GPa, transitioning to brittleness at 15 GPa; a rare behavior among halide perovskites. The electronic structure is evolved with pressure, showing a tunable band gap modulated via PBE-GGA, GGA + SOC, and HSE06 functionals; from ultraviolet-active (3.710&#xa0;eV at 0 GPa) to visible-responsive (0.822&#xa0;eV at 15 GPa) accompanied by nearly 300% increase in optical absorption. This band gap engineering enables dual-mode light harvesting, positioning it as a promising spectral-switching material for adaptive optoelectronics. Thermoelectrically, the compound exhibits ambipolar performance with high figure of merit, peaking across pressures and recovering even after a mid-range dip (0.52 at 10 GPa). Photo-catalytically, favorable band-edge alignment between 5 and 10 GPa supports solar-driven water splitting, while the onset of metallicity under extreme pressure suggests pressure-controlled catalytic switching. This study establishes K₂TlYCl₆ as a multifunctional platform with tunable optoelectronic, mechanical, and energy conversion properties, and introduces hydrostatic pressure as a novel design parameter for next generation smart perovskite-based devices.</p>