In this work, we present a density functional calculation of the structural, electronic, and mechanical properties of cubic double halide perovskites Rb₂OsX₆ (X = Cl, Br, and I). Our results show that these compounds are stable in the ferromagnetic phase with lattice parameters, bulk modulus, and their first-pressure derivatives in good agreement with other available theoretical data. The negative values of cohesive energy and formation energy, along with the absence of negative or imaginary frequencies in the phonon spectrum, confirm the mechanical stability of all the compounds. The Curie temperature (Tc) is determined using a Heisenberg model in the mean-field approximation. We obtained a half-metallic character for all compounds, making them promising materials for spintronic applications. The magnetic properties indicate that the Os atoms in all compounds are responsible for the magnetism, while the positive exchange constants suggest a strong preference for ferromagnetic alignment. This indicates a stable ferromagnetic phase and potential applications in spintronics. The mechanical properties demonstrate that the compounds studied are isotropic and ductile.