A green and sustainable approach to azo dye degradation by an electrochemically active biofilm (EAB) with Au@TiO2 nanocomposite assistance (average size of Au ~8 nm) has been developed with high efficiency and mineralization of toxic intermediates. The EAB-Au@TiO2 system degraded the dye more rapidly than the EAB without the nanocomposite, which indicated the catalytic role of the Au@TiO2 nanocomposite on the dye degradation. Toxicity measurements showed that the dye wastewater treated by the EAB-Au@TiO2 system was almost non-toxic while the dye wastewater treated by the EAB without the nanocomposite showed a high toxicity compared to the parent dye. Quantized charging and Fermi level equilibration within the Au@TiO2 nanocomposite may be attributed to the excellent catalytic activity of the nanocomposite on the dye degradation. A mechanism of the catalytic activity is also proposed. Redox behavior and quantized charging of the nanocomposite were confirmed by cyclic voltammetry (CV) and differential pulse voltammetry (DPV), respectively. The proposed protocol can be effectively utilized in wastewater treatment applications.