AbstractElucidating single‐atom effects on the fundamental properties of nanoparticles is challenging because single‐atom modifications are typically accompanied by appreciable changes to the overall particle's structure. Herein, we report the synthesis of a [Cu₅₈H₂₀PET₃₆(PPh₃)₄]²⁺ (Cu₅₈; PET: phenylethanethiolate; PPh₃: triphenylphosphine) nanocluster—an atomically precise nanoparticle—that can be transformed into the surface‐defective analog [Cu₅₇H₂₀PET₃₆(PPh₃)₄]⁺ (Cu₅₇). Both nanoclusters are virtually identical, with five concentric metal shells, save for one missing surface copper atom in Cu₅₇. Remarkably, the loss of this single surface atom drastically alters the reactivity of the nanocluster. In contrast to Cu₅₈, Cu₅₇ shows promising activity for click chemistry, particularly photoinduced [3+2] azide‐alkyne cycloaddition (AAC), which is attributed to the active catalytic site in Cu₅₇ after the removal of one surface copper atom. Our study not only presents a unique system for uncovering the effect of a single‐surface atom modification on nanoparticle properties but also showcases single‐atom surface modification as a powerful means for designing nanoparticle catalysts.