Perfluorotetradecanoic acid (PFTeDA), a long-chain PFAS, is commonly detected in aquatic environments, yet its toxicological effects remain poorly understood. Mytilus trossulus were exposed for seven days to two levels of PFTeDA, 3 and 30 ng/L. In order to assess PFTeDA toxicity towards a model species, selected physiological and biochemical biomarkers were evaluated in the digestive, respiratory and gonadal tissues. We demonstrate that already environmentally relevant levels of PFTeDA induce histopathological changes in M. trossulus, including gill oedema, digestive gland atrophy, and gonadal stimulation. These effects may impair respiration, nutrient assimilation, and reproduction, ultimately affecting individual health and population dynamics. Our findings also indicate that PFTeDA induces a mild oxidative stress response, as substantial changes in the enzymatic activities were not observed. The results highlight the role of concentration, exposure duration, and species-specific physiology in shaping oxidative outcomes. Given the ecological role of M. trossulus as a bioindicator and filter feeder, our findings add to the growing evidence of PFTeDA environmental effects.