<p>Previous findings on the link between metabolic syndrome (MetS) and the risk of all-cause dementia, Alzheimer’s disease (AD), vascular dementia (VD), and cognitive decline are inconsistent. We systematically searched Embase, PubMed, Web of Science, the Cochrane Library, and Scopus up to June 2025 for prospective cohort studies conducted in community-based settings among adults aged 18&#xa0;years or older that reported risk estimates (e.g., relative risks, hazard ratios, or odds ratios) for the association between MetS and the risk of dementia or cognitive decline. Risk of bias for studies were assessed using ROBINS-I, and the Grading of Recommendation, Assessment, Development, and Evaluation (GRADE) approach was applied to evaluate the certainty of evidence. Relative risks (RR) with 95%CI were computed using a random-effects inverse-variance method. Twenty-eight cohorts involving 6,753,197 participants were examined. MetS was significantly associated with a higher risk of all-cause dementia (RR = 1.11, 95%CI: 1.07–1.14; absolute risk difference: 2 more cases per 1000 persons), VD (RR = 1.33, 95%CI: 1.21–1.46; absolute risk difference: 7 more per 1000), and cognitive decline (RR = 1.24, 95%CI: 1.10–1.40; absolute risk difference: 5 more per 1000), all based on low-certainty evidence. No significant association was found between MetS and AD, with very low certainty. MetS components of hypertension, hyperglycemia, and low HDL-C were key risk factors, with a dose–response relationship observed between the number of MetS components and dementia risk. Subgroup analyses indicated MetS increased all-cause dementia risk in individuals under 70 (p-value for interaction: 0.048). MetS was positively associated with increased risk of all-cause-dementia, VD, and cognitive decline, but not AD. However, the effect sizes were modest and the certainty of evidence was low. Further prospective cohort studies are needed to confirm the associations.</p>