Water-soluble Cu–In–S/ZnS core/shell quantum dots with a photoluminescence quantum yield up to 38% and an emission peak tunable from 543 to 625 nm have been successfully synthesized. All of the synthetic procedures were conducted in an aqueous solution at 95 °C under open-air conditions. l-Glutathione and sodium citrate were used as the dual stabilizing agents to balance the reactivity between copper and indium ions.