Fossil fuel combustion is the primary anthropogenic source of both CO₂ and Hg to the atmosphere. On a global scale, most Hg that enters ecosystems is derived from atmospheric Hg that deposits onto the land surface. Increasing concentrations of atmospheric CO₂ may affect Hg deposition to terrestrial systems and storage in soils through CO₂-mediated changes in plant and soil properties. We show, using free-air CO₂ enrichment (FACE) experiments, that soil Hg concentrations are almost 30% greater under elevated atmospheric CO₂ in two temperate forests. There were no direct CO₂ effects, however, on litterfall, throughfall or stemflow Hg inputs. Soil Hg was positively correlated with percent soil organic matter (SOM), suggesting that CO₂-mediated changes in SOM have influenced soil Hg concentrations. Through its impacts on SOM, elevated atmospheric CO₂ may increase the Hg storage capacity of soils and modulate the movement of Hg through the biosphere. Such effects of rising CO₂, ones that transcend the typically studied effects on C and nutrient cycling, are an important next phase for research on global environmental change.