As the smallest environmental microplastics (EMPs), even at nanoscale, are increasingly present in the environment, their availability and physical and chemical effects on marine organisms are poorly documented. In the present study, we primarily investigated the uptake and accumulation of a mixture of environmental microplastics (EMPs) obtained during an artificial degradation process in early-juvenile sea bass (Dicentrarchus labrax). Moreover, we evaluated their hazardous effects using biochemical markers of cytotoxicity. Polymer distribution and composition in gill, gut, and liver were analyzed using polarized light microscopy (PLM) and Raman microspectroscopy (RMS). Our findings revealed the size-dependent ingestion and accumulation of smaller MPs (0.45–3 µm) in fish tissues even after a short-term exposure (3 and 5 days). In addition to MPs, our results showed the presence of plastic additives including plasticizers, flame retardants, curing agents, heat stabilizers, and fiber-reinforced plastic materials in fish tissues, which contributed mostly to the larger-sized range (≥ 1.2 µm). Our data showed that significant oxidative alterations were highly correlated with MPs size range. Our results emphasized that the toxicity of smaller EMPs (≤ 3 µm) was closely related to different factors, including the target tissue, exposure duration, size range of MPs, and their chemical properties.