In this study, we report a straightforward strategy for Hg²⁺ ion detection. Fluorescent Au nanoparticles (NPs) were one-pot synthesized using a polymer (polyvinyl pyrrolidone [PVP]) as both capping and fluorescence agent. The as-synthesized PVP-Au NPs showed a remarkably rapid response selectively for Hg²⁺ ions compared to 14 other metal ions. The detection limit of Hg²⁺ was estimated at 100 nM. We discuss the emission and quenching mechanism of the PVP-Au NPs, the former being attributed to metal enhanced fluorescence and the latter being related to static quenching by Hg²⁺. The fluorescence of PVP-Au NPs offers an efficient and reliable strategy for Hg²⁺ ions detection. They therefore have a great potential for applications in health and environmental monitoring.