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Ultrafine particles (≤100 nm) and other environmental nanoparticles have emerged as biologically active pollutants that can cross biological barriers, including the blood-brain barrier and the placenta. Growing evidence implicates ultrafine particles in a wide range of neuropsychiatric conditions, yet their effects remain poorly integrated into clinical and public health frameworks. In this review, we distinguish between size-defined ultrafine particles (UFPs, ≤100 nm), composition-defined environmental nanoparticles originating from combustion and secondary formation processes, and engineered nanomaterials (ENPs), which differ in physicochemical properties, exposure scenarios, and regulatory status. This narrative systematic review synthesizes findings from human and experimental studies on the neuropsychiatric and neurodevelopmental effects of environmental nanopollutants. A structured search was conducted in PubMed, Web of Science, Scopus, and Google Scholar up to November 2025, following explicit inclusion and exclusion criteria. Eligible studies included peer-reviewed human and animal research assessing mental health or neurological outcomes of nanopollutant exposure. Epidemiological studiesprimarily involving traffic-related air pollution and mixed combustion-derived ultrafine particle exposuressuggest associations with increased risk of cognitive impairment, autism spectrum disorder, depression, schizophrenia, and neurodegenerative diseases, including Alzheimer's, Parkinson's, and amyotrophic lateral sclerosis. Prenatal and early life exposures were linked to cortical thinning, altered neurodevelopmental trajectories, and early proteinopathies. Underlying mechanisms include neuroinflammation, oxidative stress, and protein aggregation. Despite methodological heterogeneity, the evidence supports the urgent need for regulation and prevention. Environmental nanopollutants constitute an under-recognized, modifiable risk factor for neuropsychiatric and neurodegenerative conditions. A paradigm shift is needed to incorporate environmental exposure history into mental health research, risk assessment, and prevention strategies. Regulatory action targeting nanopollutant emission and exposure, particularly in vulnerable populations, is critical to mitigating long-term neurological consequences.