An amino-substituted spirothiopyran, when added to an aqueous solution containing gold nanoparticles (AuNPs), promotes aggregation of AuNPs at room temperature in the dark. The sulfur atom of the spirothiopyran has high nucleophilicity due to the substitution of electron-donating amino groups and is bound to the AuNP surface via covalent Au–S binding. This promotes spontaneous spirocycle opening of the spirothiopyran and produces a positively charged indolinium moiety on the AuNP surface. This reduces electrostatic repulsion between the AuNPs and promotes their aggregation. Changing the amount of the spirothiopyran added successfully produces AuNP aggregates with tunable sizes (40–1300 nm) and narrow size distributions with standard deviation being less than 30%. This system therefore has potential to be a new simple method for self-assembly of AuNPs.