Increasing global research has identified microplastics to be impacting marine organisms. Here, we investigate the physiological and behavioral effects of thirty-six juvenile Sparus aurata exposed to microplastic enriched diets during a 21-day period. Physiological effects were assessed in liver and brain using the following biomarkers: activities of the antioxidant enzymes catalase (CAT), superoxide dismutase (SOD), glutathione peroxidase (GPx) and glutathione reductase (GRd), the detoxifying enzyme glutathione S-transferase (GST) and malondialdehyde (MDA) as indicative of lipid peroxidation. Individuals were recorded for behavior analysis (i.e. social interactions and feeding behavior). Results revealed an increased cellular stress from control to marinated-microplastic group, with virgin-microplastic group showing intermediate levels in all quantified biomarkers. Significant differences were found in liver for all biomarkers except for MDA, which suggests that the time of exposure to enriched plastic diets of this experiment is long enough to trigger the activation of antioxidant enzymes but not to produce cell damage by lipid peroxidation. In brain samples, the marinated-microplastic group presented significantly higher values for CAT and SOD, highlighting its function as primary antioxidants. Regarding behavioral effects, virgin and marinated groups displayed significantly more aggressive actions, such as biting and avoiding than control groups. In conclusion, a short period of microplastic exposure causes biomarker responses along with behavioral changes in fish. Nevertheless, further research is needed to assess long-term effects of microplastic ingestion and its potential consequences on fish populations.