AbstractAtomically precise alloy nanoclusters are one of the important nanomaterials, benefiting to investigate the structure‐property correlations at atomic level. Herein, we reported Au₁₉Cu₂(SR)₁₅(SR: adamantanethiol) alloy nanocluster. The crystal structure of Cu−Au nanoclusters was determined by X‐ray crystallography and the molecular mass was further characterized via Fourier transform ion cyclotron resonance mass spectrometer (FT‐ICR‐MS). The crystal structure shows the kernel of bi‐octahedrons Au₁₀capped by a Au₆Cu₂(SR)₉motif, a Au₂(SR)₃motif, a Au(SR)₂motif and one S atom. It was observed that two Au atoms in the longest Au₆Cu₂(SR)₉motif were replaced with two Cu atoms. Moreover, we explored optical properties of Au₁₉Cu₂(SR)₁₅by UV‐vis absorption spectroscopy and fluorescence spectroscopy. Alloy nanocluster exhibits a strong emission at 700 nm. This work provides insights into the synthesis and the enhanced properties of Au−Cu alloy nanoclusters.