By kinetically controlling a biomimetic reduction in a quasi-biological system (an aqueous solution containing electrolyte, peptide, and coenzyme), water-soluble, glutathione-capped Au clusters with a mean diameter of 1.3 nm are successfully synthesized via a new route. Opportunities that facilitate the control of the reaction are created in such a quasi-biological system. The relatively slow rate of the biomimetic reduction, the pH-dependent reducibility of the reducing agent, and the favorable structure of the capping molecules conspire together for the realization of a kinetics-controlled formation of the Au clusters under mild conditions. Compared to existing methods of synthesizing gold clusters by stoichiometrically controlling the molar ratio of the Au atoms and the ligands, our current method is based on slowing down the reduction of the Au precursors in the initial stage by adjusting the activity of the reducing agent in real time, instead of using a strong reducing agent such as NaBH4. This strategy of rationally utilizing biological or biomimetic processes with unique features to provide a beneficial complement to conventional chemical syntheses could open a new way for the sustainable development of nanotechnology.