It is important to develop simple and effective approaches for hypochlorite (OCl-) detection at trace levels due to its widespread use as a disinfectant especially for water treatments including drinking water. In this work, a simple, facile colorimetric sensor for the sensitive determination of hypochlorite was developed based on the oxidation of cysteine by OCl-, a process that prohibits the cysteine-triggered aggregation of Au nanoparticles (AuNPs) stabilized by polyvinylpyrrolidone. With this strategy, the concentration of OCl- could be detected with the naked eye and/or ultraviolet-visible spectroscopy, and the limit of detection for OCl- was 1.0 μM and 250 nM, respectively. Additionally, the proposed method shows excellent anti-interference capability against many other interfering ions and real water sample applicability.