Mass spectrometric analysis of the anionic products of interaction between platinum atomic anions, Pt−, and methane, CH4 and CD4, in a collision cell shows the preferred generation of [PtCH4]− and [PtCD4]− complexes and a low tendency toward dehydrogenation. [PtCH4]− is shown to be H−Pt−CH3− by a synergy between anion photoelectron spectroscopy and quantum chemical calculations, implying the rupture of a single C−H bond. The calculated reaction pathway accounts for the observed selective activation of methane by Pt−. This study presents the first example of methane activation by a single atomic anion.