Abstract: Nanoplastics are widely present in the marine environment and their toxicity risk increases with environmental ageing processes. However, the neurotoxic effects induced by nanoplastics in marine organisms and the mechanism of formation are not yet clear. Polyethylene terephthalate (PET) is one of the most common nanoplastics in the ocean. The environmental photoaging process can change its physicochemical properties, and then produce neurotoxic effects on marine organisms. In this study, rat adrenal pheochromocytoma cells (PC 12) were used as the model, aiming to investigate the neurotoxic effects of photoaging PET nanoplastics on marine organisms and the potential formation mechanism. The results showed that PET nanoplastics inhibited the viability and dopamine metabolism of PC 12, and the intracellular superoxide dismutase activity of PC 12 was increased by 6.73−8.24 times, glutathione peroxidase activity was decreased by 4.55−5.26 times, and the malondialdehyde content was increased by 2.03−5.12 times when the exposure concentration of light-aged PET nanoplastics was 20 μg/mL. This study may provide new insights into the ecological risk assessment of nanoplastics in the marine environment.