<p>This work presents a systematic first-principles investigation of the structural, electronic, elastic, optical, and thermoelectric properties of the cubic perovskite oxides VBiO₃ and NbBiO₃ using density functional theory within the WIEN2k framework. The calculated band structures and spin-resolved density of states confirm that both compounds exhibit metallic behavior, characterized by strong hybridization between transition-metal d states and O-2p orbitals near the Fermi level. Elastic analysis indicates that VBiO₃ satisfies the Born stability criteria and is mechanically stable and ductile, whereas NbBiO₃ shows shear instability in the cubic phase at ambient pressure due to a negative <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\({C}_{44}\)</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>C</mi> <mn>44</mn> </msub> </math></EquationSource> </InlineEquation> value. Additional pressure-dependent calculations reveal that NbBiO₃ becomes mechanically stable under hydrostatic pressure of 20 GPa, suggesting a pressure-stabilized cubic phase. The calculated optical properties demonstrate significant metallic screening and interband optical transitions, leading to pronounced dielectric response and energy-loss features in the ultraviolet energy range. Transport calculations within the BoltzTraP framework provide insight into the electronic transport behavior of these compounds; however, their metallic nature limits conventional thermoelectric efficiency. Overall, this theoretical study provides a detailed understanding of the physical properties and stability of VBiO₃ and NbBiO₃ and offers guidance for future investigations of Bi-based transition-metal perovskites under stabilized structural conditions.</p>