Although arsenic (As) groundwater contamination in South and Southeast Asia is a threat to human health, mechanisms of its release from sediment to groundwater are still not fully understood. In many aquifers, Fe­(III) minerals are often the main hosting phases for As and their stability is crucial for As mobility. Recently, a new mechanism for As mobilization into groundwater was proposed with methane (CH4) serving as an electron donor for microbially mediated reductive dissolution of As-bearing Fe­(III) minerals. To provide unequivocal evidence for the occurrence of Fe­(III)-coupled methanotrophy, we incubated sediments from an As-contaminated aquifer in Hanoi (Vietnam) anoxically with isotopically labeled 13CH4. Up to 35% of the available Fe­(III) was reduced within 232 days with simultaneous production of 13CO2 demonstrating anaerobic oxidation of 13CH4 with Fe­(III) as the electron acceptor. The microbial community at the end of the incubation was dominated by archaea affiliating with Candidatus Methanoperedens, implying its involvement in Fe­(III)-dependent CH4 oxidation. These results suggest that methanotrophs can contribute to dissolution of As-bearing Fe­(III) minerals, which eventually leads to As-release into groundwater.