Purpose: This study was done to examine the association between body mass index (BMI) and any type of clinical dementia. Methods: Participants were 60,321 people over 60 years of age enrolled in the Seoul Dementia Management Project in 2011. K-MMSE was used to classify participants as having a cognitive impairment and the Clinical Dementia Rating or DSM-IV by psychiatrists or neurologists to determine whether participants were in the dementia group or the non-dementia group. Descriptive statistics, chi-square test, and binary logistic regression analysis were performed. Results: In the univariate analysis, age, education level, living with spouse, BMI, alcohol consumption, and exercise were significantly associated with dementia. In multivariable analysis, increasing age was positively associated with dementia, and educational level was negatively associated with dementia. The exercise group had a lower prevalence of dementia than the non-exercise group. The odds ratio of dementia in the over-weight and obese groups compared to the normal group was 0.85 (95% CI 0.60, 0.98) and 0.64 (95%CI 0.46, 0.75), respectively. Conclusion: Results indicate that dementia is negatively associated with increasing BMI in people aged 60 years or older, but a prospective cohort study is needed to elucidate the causal effect relationship between BMI and dementia.