Bi₂O₃ and Ce-doped Bi₂O₃ (Ce–Bi₂O₃) nanoparticles were successfully synthesized via a green solution combustion method using gallic acid as a sustainable fuel. The synthesized materials were systematically characterized to investigate their structural, morphological, optical, electrochemical, and photocatalytic properties. X-ray diffraction confirmed the formation of phase-pure monoclinic α-Bi₂O₃, while Ce doping introduced lattice distortion without altering the crystal structure. UV-Vis analysis revealed a reduction in the optical band gap from 3.26 to 3.05 eV, indicating enhanced light absorption. Electrochemical studies in 0.1 M HCl demonstrated diffusion-controlled redox behaviour, with Ce-Bi₂O₃ exhibiting a higher proton diffusion coefficient (0.8668 × 10⁻³ cm² s⁻¹) and specific capacitance (154.89 F g⁻¹) than that of pristine Bi₂O₃. The Ce-doped electrode also showed superior electrochemical sensing performance toward ascorbic acid due to enhanced charge transfer associated with Ce³⁺/Ce⁴⁺ redox centers and oxygen vacancies. Furthermore, Ce–Bi₂O₃ achieved 96.7% degradation of malachite green dye within 10 min under UV irradiation, outperforming pristine Bi₂O₃ (82.4%) of the AR-88 dye after 90 min. The enhanced electrochemical and photocatalytic performances demonstrate that Ce–Bi₂O₃ is a promising multifunctional material for sensing and environmental remediation applications.