Lithium production has dramatically increased over the past decade, and the first cases of environmental Li pollution have been recently reported in urban and mining regions. While elevated Li concentrations may be toxic for living organisms, tools to monitor Li in the environment have not yet been developed. Consequently, its impact on key biota and human health is still poorly known. The present laboratory-based study shows that the soft tissues of blue mussels (Mytilus edulis) can be used to quantify Li contamination in coastal waters. Stable Li isotope ratios (7 Li/ 6 Li) measured in these soft tissues correlate positively with seawater Li concentrations and show precisely the threshold above which mussels shift their depuration mechanism. Combined with other data from the natural environment, the experimental results have profound implications for the fate of coastal ecosystems and shellfish consumption living under a high Li environmental level. We also highlight the need to develop innovative tools to extract Li from wastewaters before its release into rivers and, ultimately, the ocean.