The present study demonstrated the colorimetric response of biogenetic gold nanoparticles (AuNPs) to Hg2+. The colloidal AuNPs were synthesized by the cell-free extracts of yeast strain Magnusiomyces ingens LH-F1, which showed excellent selectivity to Hg2+ over other heavy metal ions, including Ag+, Cd2+, Cu2+, Ni2+, Pb2+ and Zn2+. When the biogenetic AuNPs were interacted with Hg2+ (1–500μM), a noticeable blue shift in surface plasmon resonance was observed with no significant changes in solution colour and particle size, and the absorbance ratio (A525/A532) was linearly correlated with the Hg2+ concentration in the ranges of 1–50μM and 100–500μM, respectively. High concentrations of Hg2+ (>900μM) would aggregate the nanoparticles to form Au-Hg amalgam.