Abstract <p>B-site cation replacement is a potent approach to modulate the characteristics of perovskite oxides to promote the advanced optoelectronic applications. We use the first-principles density functional theory (DFT) to explore the structural, electronic, optical, magnetic and mechanical properties of cubic terbium-based perovskites TbXO₃ (X = Ru, Os), and compare the effect of Ru, and Os directly as the B-site, in the system. The thermodynamic stability of the two compounds is confirmed by negative formation energies. Electronic structure analysis indicates a metallic nature with large electrical conductivity and optical property analysis indicates high absorption values with also significant transparency at certain wavelengths-which is a characteristic feature of high performance transparent conductors. Ferromagnetic ordering is further confirmed by spin-polarized density of states, and this implies that it can be used in many different ways. TbRuO₃ and TbOsO₃ are both ductile and robust as measured by mechanical tests. Together, the combination of metallic conductivity, moderate optical transparency, magnetic, and mechanical resilience makes B-site-engineered TbXO₃ perovskites strong candidates in the next-generation optoelectronic and photovoltaic devices.</p>