<p>In this study, we explore the structural, electronic, optical, and thermoelectric properties of rare-earth halide double perovskites Cs₂ErXCl₆ (X = Ag, Au) using the modified Becke–Johnson (mBJ) approach within the WIEN2k framework. The stability of both perovskites was confirmed through multiple stability parameters, including tolerance and octahedral factors, as well as their negative formation enthalpy values. The formation enthalpies of Cs₂ErAgCl₆ and Cs₂ErAuCl₆ are calculated as −&#xa0;2.29 eV and −&#xa0;2.0 eV, respectively, indicating their thermodynamic stability. Additionally, their tolerance factor values of 0.88 and 0.87 further confirm their structural stability. The mechanical properties demonstrated the stability of both perovskites, as they both satisfied the Born stability criteria. The Cauchy pressure values of 3.54 GPa for Cs₂ErAgCl₆ and 4.09 GPa for Cs₂ErAuCl₆ indicate ductile nature, with greater softness in Cs₂ErAuCl₆. The semiconductive nature for both HDPs is validated through band structures graphs with bandgap values of 1.07 and 1.50 eV for Cs₂ErAgCl₆ and Cs₂ErAuCl₆, respectively. Both materials show excellent optical transparency with low reflectivity, starting at 0.64 for Cs₂ErAgCl₆ and 0.53 for Cs₂ErAuCl₆, indicating their suitability for high-transmittance applications. As a result, the material system has a significant transmittance and could be utilized for high-transmittance application. Furthermore, thermoelectric results reveal p-type behavior in Cs₂ErAgCl₆ and n-type behavior in Cs₂ErAuCl₆ based on Seebeck coefficient trends. These findings highlight Cs₂ErXCl₆ (X = Ag, Au) as promising multifunctional materials for optoelectronic and thermoelectric technologies.</p>