AbstractSimilarly to enzymes, functionalized gold nanoparticles efficiently catalyze chemical reactions, hence the term nanozymes. Herein, we present our results showing how surface‐passivated gold nanoparticles behave as synthetic nanonucleases, able to cleave pBR322 plasmid DNA with the highest efficiency reported so far for catalysts based on a single metal ion mechanism. Experimental and computational data indicate that we have been successful in creating a catalytic site precisely mimicking that suggested for natural metallonucleases relying on a single metal ion for their activity. It comprises one Zn(II) ion to which a phosphate diester of DNA is coordinated. Importantly, as in nucleic acids‐processing enzymes, a positively charged arginine plays a key role by assisting with transition state stabilization and by reducing the pKa of the nucleophilic alcohol of a serine. Our results also show how designing a catalyst for a model substrate (bis‐p‐nitrophenylphosphate) may provide wrong indications as for its efficiency when it is tested against the real target (plasmid DNA).