Abstract Double perovskite optoelectronic devices are gaining popularity due to their advantageous features, such as an efficient and easy-to-manage crystalline structure. In this study, we looked into the optoelectronic, mechanical, and thermoelectric attributes of Cs₂MScCl₆ (with M being Na or K) using density functional theory. The obtained results show that both compounds can exist stably in cubic double perovskite structure, space group Fm 3 ¯ m . The electronic band structures of Cs₂NaScCl₆ and Cs₂KScCl₆ demonstrate a direct semiconducting band gap of 3.829 eV and 3.996 eV, respectively. As the pressure rises up to 200 GPa, Cs₂KScCl₆ reveals extraordinary tunability along with an indirect band gap of 2.000 eV. Moreover, optical analysis—comprising dielectric constant calculations, absorption coefficient measurements, refractive indices checks, reflectivity assessments, energy loss estimations, and electrical conductivity appraisals—shows promising results too. Specifically, the absorption coefficient and conductivity predominantly fall within the ultraviolet range under normal atmospheric pressure. The thermoelectric characteristics suggest a noteworthy Seebeck coefficient, low thermal conductance and good figure of merit implying it could be used in the domain of sustainable energy.