Hyperhalogens were recently identified as a new class of highly electronagative species which are composed of metals and superhalogens. In this work, high-level theoretical calculations and photoelectron spectroscopy experiments are systematically conducted to investigate a series of coinage-metal-containing hyperhalogen anions, Cu(BO2)2−, Ag(BO2)2−, and Au(BO2)2−. The vertical electron detachment energy (VDE) of Ag(BO2)2− is anomalously higher than those of Au(BO2)2− and Cu(BO2)2−. In quantitative agreement with the experiment, high-level ab initio calculations reveal that spin–orbit coupling (SOC) lowers the VDE of Au(BO2)2− significantly. The sizable magnitude of about 0.5 eV of SOC effect on the VDE of Au(BO2)2− demonstrates that SOC plays an important role in the electronic structure of gold hyperhalogens. This study represents a new paradigm for relativistic electronic structure calculations for the one-electron-removal process of ionic AuIL2 complexes, which is characterized by a substantial SOC effect.