ABSTRACT The ubiquity of polystyrene nanoplastics (PS-NPs) in our environment raises substantial concerns about their potential impact on human health. Recent studies have shown that PS-NPs cross the blood-brain-barrier and accumulate in the central nervous system (CNS), raising the concerns on the causal role of PS-NP exposure and neurodegenerative diseases. In this study, we utilized human-induced pluripotent stem cell-derived neurons to examine the effects of PS-NPs on neuronal function and health. Our results revealed that PS-NPs penetrate neuronal cells in a size-dependent manner, and are bound by various cellular proteins, including TDP43, a key protein implicated in amyotrophic lateral sclerosis (ALS). Interestingly, CK1 and GSK3β kinases that are known to phosphorylate TDP-43 were found to be associated with PS-NPs. This observation suggests that PS-NPs may play a role in facilitating conditions that lead to TDP43 phosphorylation. We further demonstrate that exposure to healthy motor neurons with PS-NPs resulted in ALS-like phenotypes, characterized by hyperphosphorylated TDP-43, disrupted neuronal morphology, impaired mitochondrial respiration, and accelerated motor neuron death. These findings suggest that PS-NPs contribute to the pathogenesis of neurodegenerative diseases such as ALS and highlight the urgent need for strategies to limit human exposure to nanoplastics.