The COVID-19 pandemic has triggered an unprecedented need for single-use face masks, leading to an alarming increase in plastic waste globally. Consequently, the improper disposal of face masks has added to the existing burden of plastic pollution in the oceans. However, the complete environmental and marine ecotoxicological impact remains unclear. This study aims to investigate the ecotoxicological impact caused by the weathering of disposable face masks (DFMs) in the marine environment on mussels Mytilus galloprovincialis (M. galloprovincialis ) by assessing biochemical, cytotoxic, and genotoxic effects. The mask leachate was analysed for the presence of nano and microplastics. Furthermore, the leachate was used in in vivo and in vitro toxicity bioassays to assess its impacts on M. galloprovincialis . The in vivo exposure of M. galloprovincialis to face mask leachate for 14 days induced a significant increase in catalase (CAT) activity in mussel gills, although not enough to prevent oxidative damage to cell membranes. DNA damage was also registered in mussel haemocytes after in vivo exposure to mask leachate. The in vitro Neutral Red (NR) cytotoxicity assay indicated that leachate concentrations ≤ 0.5 g/L-1 pose a significant risk to the health of mussel haemocytes, which seems a reliable tool for the cytotoxicity impact assessment of face masks in the marine environment. Therefore, the leachate obtained from face masks in seawater causes oxidative stress, oxidative damage, cytotoxicity, and genotoxicity in M. galloprovincialis , indicating that the plastic burden generated by DFMs in the ocean and its subsequent weathering represents a ubiquitous and invisible threat to the marine biota.