<p>Plastic debris has recently been identified as a hotspot for abiotic metal transformations, triggered primarily by photo-weathering under sunlight. Here we perform a set of experiments with freshwater in the field and laboratory to explore metal transformations under dark conditions. We demonstrate that light-independent weathering of plastics leads to methylation of inorganic mercury (Hg(II)) in waterbodies. We propose that methylation occurs via an abiotic pathway involving three chain reaction steps, namely the release of plastic-derived dissolved organic matter (P-DOM), complexation of P-DOM with Hg(II) and intramolecular transfer of methyl groups. P-DOM is released during the light-independent oxidation of plastics via reactive oxygen species. Density functional theory simulations confirm the thermodynamic feasibility of the intramolecular transfer of methyl groups to Hg(II), upon its complexation with oxygen-containing groups in P-DOM. Model estimates demonstrate that polypropylene in freshwater produces methylmercury via this abiotic pathway with Hg(II) methylation potentials from 2.8 × 10⁻⁵% per day to 5.5 × 10⁻²% per day in China and 4.0 × 10⁻⁶% per day to 7.5 × 10⁻³% per day in other regions of the world. Plastic debris is therefore a hidden driver of abiotic methylmercury formation in dark waters. Our study uncovers a pathway through which the ongoing plastic pollution alters mercury cycling, posing a burgeoning threat to planetary health.</p>