<p>Double perovskites have emerged as a focal point in materials research due to their intriguing properties. In the present study, first-principles calculations within the framework of density functional theory were employed to investigate the structural, electronic, thermodynamic, and mechanical properties of Ba₂RbXO₆ (X = Cl, Br), double perovskites. The structural stability was confirmed from the values of tolerance and octahedral factors. The negative formation energies proved the thermodynamic stability. The real and positive phonon dispersion curves affirmed the dynamical stability. The analysis of electronic properties demonstrated the metallic nature due to the overlapping of valence and conduction bands. The quasi-harmonic Debye approximation was utilized to study the effect of temperature and pressure on thermodynamic properties such as bulk modulus, Debye temperature, entropy, specific heat at constant volume (Cv), and thermal expansion. Where, Cv obeyed the Debye model for both the double perovskites up to approximately 250&#xa0;K, beyond which it followed the Dulong-Petit law (Cv = 3R). The investigation of mechanical properties revealed stiffness, ductility, and anisotropy for both studied perovskites.</p>