Real-time monitoring of reaction processes is helpful for understanding the reaction mechanisms. In this study, we investigated the etching mechanism of gold nanoparticles (AuNPs) by iodine on a single-nanoparticle level because AuNPs have become important nanoprobes with applications in sensing and bioimaging fields owing to their specific localized surface plasmon resonance (LSPR) properties. By using a scattered-light dark-field microscopic imaging (iDFM) technique, the in situ KI/I₂-treated etching processes of various shapes of AuNPs, including nanospheres (AuNSs), nanorods (AuNRs), and nanotrigonal prisms (AuNTs), were monitored in real time. It was found that the scattered light of the different shapes of AuNPs exhibited noticeable color changes upon exposure to the etching solution. The scattering spectra during the etching process showed obvious blue-shifts with decreasing scattered intensity owing to the oxidation of Au atoms into [AuI₂]^–. Both finite-difference time-domain (FDTD) simulations and monitoring of morphological variations proved that the etching was a thermodynamic-dependent process through a chamfering mechanism coupled with layer-by-layer peeling, resulting in isotropic spheres with decreased particle sizes.