We observe that etching approximately 6 nm spherical gold nanoparticles (AuNPs) with cysteine produces assembled AuNPs with approximately 2 nm of particle size, which specifically exhibit a new absorption band around 377 nm. The absorption band possesses an optical activity reflecting the chirality of cysteine attached on the AuNPs. We also observe that the decrease in the chiral purity of cysteine used from 100% to 60% enantiomeric excess (ee) enhances the optical rotation of the plasmonic band. The anomalous enhancement of the optical rotation is caused by the recovery of the net optical rotation of the assemblies of the AuNPs because the decrease in the chiral purity of cysteine decreases the size of the disordered assemblies. Since disordered assemblies cancel their inherent optical rotation of the AuNP components, the decrease in their size recovers the net optical rotation of the assemblies. Our results demonstrate that control of the chiral purity of optically active thiols etching AuNPs is a potential method to fabricate assemblies of AuNPs with unique optical activity.