<p>The development of efficient thermoelectric materials for direct waste heat–to–electricity conversion remains a major challenge, particularly for high-temperature applications. In this paper, a systematic first-principles examination of the structural, electronic, elastic, thermal and thermoelectric characteristics of double perovskite oxides, Ba₂<i>X</i>ReO₆ (<i>X</i> = Li, Rb, Cs), is conducted using the density functional theory, alongside Boltzmann transport theory. Structural optimization proves that all compounds are thermodynamically stable in the cubic Fm3̅m phase with negative formation energies. Furthermore, ab initio molecular dynamics simulations performed at 300&#xa0;K confirm the dynamical stability of the optimized structures. The electronic structure analysis reveals semiconducting behavior in Ba₂CsReO₆, Ba₂LiReO₆, and Ba₂RbReO₆, all exhibiting indirect and narrow band gaps, which are ideal characteristics for thermoelectric transport. The elastic constants obtained and used have met the Born stability requirements, indicating mechanical stability and ductile behavior. A systematic decrease in Debye temperature, melting temperature, and lattice thermal conductivity with increasing A-site ionic radius led to ultralow lattice thermal conductivity of Ba₂RbReO₆ and Ba₂CsReO₆. Thermoelectric transport calculations show positive Seebeck coefficients and enhanced power factors at elevated temperatures. Of the investigated compounds, the Ba₂RbReO₆ has the largest thermoelectric figure of merit (ZT), owing to an optimum balance between electrical conductivity and reduced thermal conductivity. Overall, our results suggest that Ba₂<i>X</i>ReO₂, particularly Ba₂RbReO₂, are promising candidates for high-temperature thermoelectric applications.</p> Graphical Abstract <p></p>