Arsenic groundwater contamination is threatening the health of millions of people worldwide, particularly in river deltas in South and Southeast Asia where geogenic arsenic is released from sediments1-5. In most cases, the release of arsenic (As) was shown to be caused by microbially catalyzed reductive dissolution of As-bearing Fe(III) (oxyhydr)oxide minerals with organic carbon being used as microbial electron and energy source6-8. Although in many As-contaminated aquifers high concentrations of methane (CH4) were observed9-11, the role of CH4 for As mobilization is unknown. Here we demonstrate that CH4 functions as electron donor for methanotrophic microorganisms and triggers the reductive dissolution of As-bearing Fe(III) (oxyhydr)oxide minerals leading to As mobilization. In microcosms with As-bearing sediments from the Red River Delta amended with environmentally relevant concentrations of CH4 we found that CH4 triggers Fe(III) mineral reduction, supports the growth and activity of type-1 aerobic methanotrophs and archaea affiliating with Candidatus Methanoperedens, increases the abundance of methane oxidation mcrA and pmoA genes, and ultimately mobilizes significant amount of As into the water. Our findings provide evidence for a methane-mediated mechanism for arsenic mobilization that is distinct from previously described pathways. Taking this together with the common presence of methane in arsenic-contaminated aquifers, we suggest that this methane-driven arsenic mobilization may contribute to arsenic contamination of groundwater on a global scale.