Reaction of the atomic gold anion (Au-) with CH3I under high-pressure helium gas affords the adduct AuCH3I-. Photoelectron spectroscopy and density functional theory calculations reveal that in the AuCH3I- structure the I and CH3 fragments of CH3I are bonded to Au in a linear configuration, which can be viewed as an oxidative addition product. Theoretical studies indicate that oxidative addition proceeds in two steps: nucleophilic attack of Au- on CH3I, followed by migration of the leaving I- to Au. This mechanism is supported by the formation of an ion-neutral complex, [Au-...t-C4H9I], in the reaction of Au- with t-C4H9I because of the activation barrier along the S(N)2 pathway resulting from steric effects. Theoretical studies are conducted for the formation mechanism of AuI2-, which is observed as a major product. From the thermodynamic and kinetic viewpoints, we propose that AnI(2)(-) is formed via sequential oxidative addition of two CH3I molecules to Au-, followed by reductive elimination of C2H6. The results suggest that Au- acts as a nucleophile to activate C(sp(3))-I bond of CH3I and induces the C-C coupling reaction of CH3I.