Double perovskites that are lead-free are becoming increasingly popular for use in a variety of applications such as solid-state lighting, thermoelectrics, and photovoltaics. In this study, the physical properties of inorganic double perovskites Rb2NaBiX6 (X= Cl, F, Br, I) were thoroughly examined using the density functional theory (DFT) based Wien2K code. Structural parameters were determined using the Murnaghan equation of state and Chapin’s method, which were found to be in good agreement with other current results. The research focused on investigating the mechanical, electronic, optical, and transport properties of Rb2NaBiF6, Rb2NaBiCl6, Rb2NaBiBr6, and Rb2NaBiI6 double perovskites for use in optoelectronics and photovoltaic devices. The calculated indirect band gap values using GGA and LDA approximation were found to be 4.932, 3.637, 2.972, and 2.002 eV for Rb2NaBiF/Cl/Br/I6, respectively. Optical properties were analyzed in terms of dielectric constants, absorption coefficient, refractive index, and reflectivity. The highest peak absorption was found to occur in the ultraviolet and visible ranges for all compounds, making them suitable for use in optoelectronic applications. The electron transport properties were studied by examining electrical conductivity, thermal conductivity, Seebeck coefficient, and figure of merit. The resulting ZT values obtained by the calculations in the temperature range of 100–1200K were found to be 0.82/0.81 for materials I6/Cl6 and 0.80/0.78 for materials F/Br. Furthermore, the analysis of transport properties suggested that the inorganic double perovskites Rb2NaBiX6 (X= Cl, F, Br, I) have p-type semiconducting characteristics.