<p>Driven by the growing demand for efficient energy conversion and waste heat recovery technologies, the exploration of novel thermoelectric materials with high performance and stability has become a critical research focus. In this study, first-principles density functional theory (DFT) calculations were employed to systematically investigate the structural, electronic, elastic, and thermoelectric properties of cubic double perovskites Sr₂MReO₆ (M = Li, Na, K). All three compounds crystallize in the Fm3̅m space group and are found to be thermodynamically and mechanically stable. Ab initio molecular dynamics (AIMD) simulations were further carried out and confirm the dynamical stability of these materials. Electronic band structure analysis reveals direct semiconducting behavior, primarily governed by strong Re–O hybridization and M-site cation–dependent interactions. Elastic property analysis indicates that Sr₂LiReO₆ and Sr₂KreO₆ exhibit brittle behavior, whereas Sr₂NaReO₆ shows ductile characteristics. Thermoelectric evaluation demonstrates that the Seebeck coefficient and power factor increase with temperature, with Sr₂KreO₆ achieving a maximum dimensionless figure of merit (ZT) of approximately 2.0 at 1000&#xa0;K. These findings highlight Sr₂KreO₆ as a promising candidate for high-temperature thermoelectric applications and emphasize the tunability of physical properties through strategic M-site cation substitution, suggesting potential use in energy harvesting, waste heat recovery, and high-temperature electronic devices.</p>