Abstarct: Abstract In this research, the electronic structure, and optoelectronic properties of Cs2InMX6, a type of double perovskite compound were explored. The approach utilized quantum calculations baxsed on Density Functional Theory (DFT) employing both Local Density Approximation (LDA) and General Gradient Approximation (GGA). Additionally, for the ascertainment of precise band gap values, the HSE06 exchange-correlation functional was applied. These calculations were conducted using the Quantum Espresso computing package, specifically through the PWSCF code, with a Plane-Wave (PW) basis set and pseudopotentials (PP). Furthermore the effect of spin-orbit interaction corrections on the band structures was meticulously analyzed. Initially, leveraging LDA and GGA approximations, the most suitable method for optimizing the structure and identifying the stable phase of Cs2InAgCl6, Cs2InBiCl6, and Cs2InSbCl6 was determined. This analysis yielded data on lattice constants, bulk modulus, optimal unit cell volume, and ground state energies. Moreover, to delve into the electronic structures of these compounds, various parameters were calculated and analyzed. These included the Density of States (DOS), Partial Density of States (PDOS), electron charge density, bond types, Fermi level positioning, conduction and valence band edges, and band gap values. The effects of cation transmutation on both structural and electronic properties, and compound’s band gap size alterations, were also explored, proving invaluable for bandgap engineering. This included examining cationic alloying in both M and X positions of halogen to identify structures with optimal physical characteristics for optoelectronic uses. The investigation extended to the optical properties of these compounds, encompassing the real and imaginary components of the dielectric function, absorption coefficients, refractive indices, and extinction coefficients, culminating in a comprehensive analysis of the results. The studies confirmed that these compounds are direct bandgap semiconductors, exhibiting efficient optical absorption in the visible and ultraviolet regions. This characteristic underscores their potential in solar cell technologies as absorbent laxyers and in various other optoelectronic applications.