Electrodes with nanostructures offer a better avenue to accomplish high performance supercapacitors. In view of this, seaweed like nanostructures of Bi2O3 was synthesized by electrochemical anodization of bismuth film in oxalic acid electrolyte. Even though, as-anodized films were amorphous, annealing promoted the crystallinity and promoted a phase transformation from β to γ-Bi2O3 on sequential annealing. Raman spectroscopy supported the XRD results of diminishing beta-phase Bi2O3 on sequential annealing, structural distortion and phase transformation. HRSEM images displayed the Codium tomentosum seaweed like nanostructure embedded with multiple nanofibers and nanoparticles. Cyclic voltammetry exhibited a maximum specific capacitance of 331Fg−1 for β-Bi2O3 electrode. The performance of Bi2O3 electrode pseudocapacitor depends on its phase crystallinity, morphology and charge carrier density. Impedance spectral analysis revealed that the increase in the specific capacitance was mainly owing to the decrease of charge transfer resistance and the fast ion diffusion in the electrode.