Ultrasmall atomically precise monolayer-protected gold thiolate nanoclusters are an intensely researched nanomaterial framework, but there is a lack of a system that can be directly synthesized and undergo interfacial surface chemistry. We report an [Au-25(SCH2CH2-p-C6H4-N-3)(18)](-) nanocluster platform with azide moieties appended onto each surface ligand. The structure of this surface reactive cluster has been confirmed by single-crystal X-ray crystallography, mass spectrometry and ultraviolet visible, infrared and nuclear magnetic resonance spectroscopies. We show that all surface azide moieties are amenable to cluster-surface strain-promoted alkyne-azide cycloaddition chemistry with a strained cyclooctyne, opening this as a new platform to allow functional, postassembly surface modifications to this very prominent nanocluster.