Microplastics (MPs) are important exempla of the Anthropocene and are exerting an increasing impact on Earth’s carbon cycle. The huge imbalance between the MPs floating on the marine surface and those that are estimated to have been introduced into the ocean necessitates a detailed assessment of marine MP sinks. Here, we demonstrate that cold seep sediments, which are characterized by methane fluid seepage and a chemosynthetic ecosystem, effectively capture and accommodate small-scale (< 100 μm) MPs, with 16 types of MPs being detected. The abundance of MPs in the surface of the sediment is higher in methane-seepage locations than in non-seepage areas. Methane seepage is beneficial to the accumulation, fragmentation, increased diversity, and aging of MPs. In turn, the rough surfaces of MPs contribute to the sequestration of the electron acceptor ferric oxide, which is associated with the anaerobic oxidation of methane (AOM). The efficiency of the AOM determines whether the seeping methane (which has a greenhouse effect 83 times greater than that of CO₂ over a 20-year period) can enter the atmosphere, which is important to the global methane cycle, since the deep-sea environment is regarded as the largest methane reservoir associated with natural gas hydrates.