Adsorption gel was prepared by carbonizing a residual waste of microalgae generated in the biofuel conversion process. The prepared gel exhibited high affinity and selectivity for Au (III) over other precious and base metal ions in hydrochloric acid. From the adsorption isotherms, the maximum loading capacity of Au (III) on the carbonized microalgae was evaluated as 1.20 mmol/g (236 mg/g), which is much higher than that on the original microalgal residue. Observations by microphotographs, scanning electron microscope, and X-ray diffraction confirmed the formation of metallic Au, suggesting that a redox reaction had taken place between the adsorbent and Au (III) ions during adsorption. Adsorption kinetics was interpreted in terms of pseudo-first order rate expression to evaluate the activation energy of this system as 33.7 kJ/mol.