Polystyrene micro/nanoplastics (MPs/NPs) are globally recognized environmental concerns due to their widespread pollution and detrimental effects on physiological functions. However, the neurotoxic effects and underlying mechanisms of MPs/NPs on brain function in adolescents remain incompletely understood. This study investigated the effects of polystyrene MPs/NPs on neurobehavioral function in adolescent mice, utilizing multi-omic analysis and molecular biology assays to explore potential mechanisms. Following oral exposure of MPs (5 μm) or NPs (0.5 μm) at 0.5 mg/day for 4 weeks, NPs induced more severe cognitive impairment in mice than MPs, as assessed by the Morris water maze and Y-maze tests. This impairment might be associated with the neuron loss and neurogenesis inhibition caused by NPs, while dendritic spine loss mediated by MPs in the hippocampus. Furthermore, analysis of hippocampal transcriptome and Western blotting indicated the potential involvement of the PI3K/AKT pathway in NPs-induced neurotoxicity. Meanwhile, exposure to NPs induced more pronounced disruptions in the hippocampal metabolome and gut microbiota, and strong correlations were observed between changes in hippocampal metabolites and gut bacteria. This study elucidated the toxicity mechanism of MPs and NPs in inducing cognitive impairment in adolescent mice, providing insights into their toxicological impacts and potential strategies for intervention.