Previous studies have indicated increased dementia risk associated with fine particulate matter (PM(2.5)) exposure; however, the findings are inconsistent. In this systematic review, we assessed the association between long-term PM(2.5) exposure and dementia outcomes using the Burden of Proof meta-analytic framework, which relaxes log-linear assumptions to better characterize relative risk functions and quantify unexplained between-study heterogeneity (PROSPERO, ID CRD42023421869). Here we report a meta-analysis of 28 longitudinal cohort studies published up to June 2023 that investigated long-term PM(2.5) exposure and dementia outcomes. We derived risk-outcome scores (ROSs), highly conservative measures of effect size and evidence strength, mapped onto a 1-5-star rating from 'weak and/or inconsistent evidence' to 'very strong and/or consistent evidence'. We identified a significant nonlinear relationship between PM(2.5) exposure and dementia, with a minimum 14% increased risk averaged across PM(2.5) levels between 4.5 and 26.9 µg m(-3) (the 15th to 85th percentile exposure range across included studies), relative to a reference of 2.0 µg m(-3) (n = 49, ROS = 0.13, two stars). We found a significant association of PM(2.5) with Alzheimer's disease (n = 12, ROS = 0.32, three stars) but not with vascular dementia. Our findings highlight the potential impact of air pollution on brain aging.