Metal nanoclusters have emerged as promising near-infrared (NIR)–emissive materials, but their room-temperature photoluminescence quantum yield (PLQY), especially in solution, is often low (<10%). We studied the photophysics of Au₂₂(^tBuPhC≡C)₁₈ (Au₂₂) and its alloy counterpart Au₁₆Cu₆(^tBuPhC≡C)₁₈ (Au₁₆Cu₆) (where ^tBu is tert-butyl and Ph is phenyl) and found that copper (Cu) doping suppressed the nonradiative decay (~60-fold less) and promoted intersystem crossing rate (~300-fold higher). The Au₁₆Cu₆ nanocluster exhibited >99% PLQY in deaerated solution at room temperature with an emission maximum at 720 nanometers tailing to 950 nanometers and 61% PLQY in the oxygen-saturated solution. The approach to achieve near-unity PLQY could enable the development of highly emissive metal cluster materials.