Nowadays, there exists an increasing trend of investigating ternary perovskite oxide materials because of their wide applications in solar cells and similar electronic and optoelectronic devices. This work describes a systematic study performed through state-of-the-art ab initio density functional theory (DFT) approach associated with the magnetic, electronic, and thermoelectric behavior of XTaO3(X = alkali metals i.e., Rb, Fr) tantalate perovskite oxides. These compounds are found to be semiconductor because both materials have shown significant values of the band gap energy, that is, indirect energy band gap of 1.12 eV (up arrow down arrow) in RbTaO(3)and direct band gap of 1.08 eV (up arrow down arrow) in FrTaO3. The calculated formation energies appear to be decreasing such as -3.54 and -1.92 eV for RbTaO(3)and FrTaO(3,)respectively. The analysis through projected density of states discloses that Fr-6pand 2pstates from anion sites have shown overall contribution in forming the highest edge of the valence band (VBM), while, Ta-5dstates have depicted chief role to form major part of the conduction band minima, thereby, contributing in electronic conductivity. The much better values of thermoelectric parameters such as power factor of (5.5062 x 10(13) W.m(-1).K-2.s(-1)) and figure of merit (0.82) demonstrate that RbTaO(3)is more dominating and promising semiconducting material which would, comparatively, be deemed an appropriate candidate for application in developing efficient thermoelectric devices. Finally, on the bases of above cited facts and the already existing data on similar perovskite oxides, it can be speculated that the studied compounds seem potential candidates to understand underlying physics and for possible applications in thermoelectric and other allied devices.